Flash allocates memory in chunks, so it may allocate more than it requires. Also if you app has no memory leaks the total memory being used will reach a peak (depending upon your app) and GC will kick in if the app needs more memory.
Setting variable to null does not guarantee that GC will run. You should use the profiler in flex 3 to investigate if there are any memory leaks. Also the profiler has a button by which you can invoke a GC run. Thanks, Gaurav
