Hi
There are several factors to this problem/behavior.
The bars do not specify exact the assisiation speed to which you are connected, but give you an indication.
Head over to the dashboard on the controller, find the devices and compare the speed you are assosiated to the AP.
At the same time, Apple devices tend to be a bit more sticky to it's "home" AP than other devices, but usually they roam as other devices.
One important thing to share is wether you are using AP-225 or not. I have seen extensive problems with AP groups that were made for older AP's (mainly AP-130), when the customer upgraded to AP-225.
In this scenario, we had to make seperate AP groups for the AP-22x, and also did a bit of tweeking with the help of Aruba.
The problem was not only with iPhone, but other devices aswell. But Apple devices was more often affected.
Roar Fossen