This is inherent to all floating bridges and is perfectly normal. If you watch the bridge as you bend a string, it will pull forward, and as it does every other string is pulling flat. There are a few ways to resolve this situation. The first is to micro bend the double stop into tune, a technique that's very easy to get used to [for me I generally pull the B string while pushing on the G, just feels more natural than pushing both to different degrees. Doesn't work so well on the E/B though as you'll pull the E off the frets ;)] There are also devices such as the Tremsetter and Ibanez Backstop, that both address the problem. Another cure is to add spring tension that's pulling on the bridge so it takes much more force to pull the bridge forward with a string bend. It's something you'll need to experiment with to find the degree of relief you can live with. You can add one spring or even two, and with 5 springs you'll have lots of extra tension "holding" the bridge in position. making it much more difficult to pull forward. Unfortunately adding all this tension makes for a very stiff playing guitar as part of that buttery feel on string bending is the fact the bridge *is* pulling forward with little resistance, but, it is a solution to keep your double stops in tune.
To really stiffen it up 5 springs would almost give hardtail-esque performance, but because the springs are not counterbalancing heavy gauge strings it means the springs will be very closed with the strings at pitch, So closed you'll get very stiff string bending, a very heavy bar feel, and they're also an absolute pain to make small trem angle adjustments, one of the things you'll have to learn to deal with while performing maintenance. You might find that just a fourth spring is enough to be tolerable with how well it will hold tune against a bend but still give you more pliable action. Even 3 using an arrow formation will add more tension. The point is you'll have to experiment to find a solution that's right for you.
So far I've been speaking of stock Ibanez springs which you'll find in nearly all new Ibanez. There will be 3 and they are 52mm long. They do make a short 47mm spring that is about as strong as 2 of the 52mm springs. Currently they are in K7 models and I'll run into them in some Prestige and J Custom models, read the section on Sensitive Springs. Joe has always used 2 of these 47mm strings in arrow formation as this was his preferred combination of keeping double stops more in tune vs. feel. I have used 2 springs in arrow formation to counterbalance 10 gauge string in regular tuning. 3 of these short springs in straight formation will counterbalance 12's. They offer the highest tension you can put in your guitar but you will pay the price in maintenance as they are extremely sensitive making adjustments [again, read the Sensitive Springs section]. For no logical reason have I ever used more than 3 x 47's but if you want a "hardtail" floating bridge I'm sure 5 would get you there! ;)
A read through the Trem Angle section and the Removing The Bridge section should get you prepped for the task. With the guitar in tune and the nut locked, takeoff the string lock block. There are now 5 holes in the trem block, and 5 hooks on the spring claw. If it's factory you have 3 springs in the 1 3 5 holes/hooks with the spring lock on. To add springs you'll have to use the holes for the spring lock [for an alternative read the last paragraph]. For a 5 spring setup just add springs to the 2 and 5 holes/hooks. To use 4 springs you'll be using 1 2 4 5 leaving the center hole/hook empty. These are all straight spring setups. An angled setup would be 2 springs in the 1 and 5 hole on the trem to the 2 and 4 claw and the center spring straight [or absent, depending on the results you're trying to achieve]
After you've changed tension in whatever formation you're experimenting with, he guitar was in tune when you started so now it's just a matter of adjusting the claw so the the total spring tension matches the string tension at pitch. In other words, loosen the claw springs until the guitar is in tune again, using the claw springs as your tuners. I always keep my claw parallel to the cavity instead of at any weird angle [which some people believe affects the feel of their guitars and why they do it, but that's just more experimenting you're free to do if you want to spend the time ;)] You're not going to get the guitar into perfect tune this way but you can get extremely close. Just remember that the springs will adjust to their new memory so stop short of where you want to be and the adjustment will take it further, let rest for an hour, tweak into perfection and lock and fine tune to play. If you just want to quickly test the feel you can do a half assed setup getting it close to tune as possible, lock the nut and you'll get a rough idea if you want to fine tune it more because it's close to what you want, or you may know instantly you want a harder hold, weaker, whatever you're trying to achieve with the change. Only you will know when it feels right and performs to your needs so taking the time to experiment and find the best setup for you is time well spent. Me, I happen to LOVE the fact the bridge pulls forward making string bending a breeze, to me that's one of the best benefits of the floating system ;o}
If you're a mad crazy whammy user with violent tendencies toward monster pull-ups you might want to consider using double eyelet springs [which have the eyelet on both ends instead of just one] for any additional added springs, this way you can still use the spring lock block to keep a spring from popping off [in the rarest occasion they would]