Well I just ripped an 11 track cd at 256 with iTunes 7.5 in Leopard. The actual bit rates of the songs vary from 252 to 260.
That is by design. All AAC files encoded by iTunes are ABR, which gives it the ability to use a slightly bigger bitrate for a frame if necessary. iTunes until recently only showed your target bitrate (which is wrong), not the actual bitrate (which is the correct way to show it).
Also if you select the VBR option, the bitrate differences of course will be larger.