I can confirm your experience: IETester sometimes deviates from what the “real” browser shows. I have seen this even with pure CSS constructions in simulated IE7 and IE8.
I have no own experience with them, but MS offer virtual machine packages for the various versions of IE. They migh be worth a shot, I’ve been planning to give them a test drive for a while. They run their native browsers and are bound to be 100% reliable.