Cocoa Memory Issue，当从数组中删除对象时，内存不会被回收

I have a retain/release project that has a simple algorithm. I start out w/ a 100000 objects mutable array and every 5 seconds interval I remove 1000 objects at the beginning and add 1000 objects at the end. In theory my memory footprint should stay the same after a lil delay, however it consistently rises until it caps out at a certain amount. But removing all its objects with "[array removeAllObjects]" and releasing the array doesn't reclaim all the memory back, just a portion. I am running in release scheme, with no debugger, and using the activity monitor to track memory usage. I have a retain/release project that has a simp