I think it may be ASLR at work. You can turn off DEP to test this theory.
See here for a C# utility class to check memory information: https://stackoverflow.com/a/8716410/552139
By the way, with this tool, I found that the difference between the maximum and minimum stack size is around 2 KiB, which is half a page. That’s weird.
Update: OK, now I know I’m right. I followed up on the half-page theory, and found this doc that examines the ASLR implementation in Windows: http://www.symantec.com/avcenter/reference/Address_Space_Layout_Randomization.pdf
Quote:
Once the stack has been placed, the initial stack pointer is further
randomized by a random decremental amount. The initial offset is
selected to be up to half a page (2,048 bytes)
And this is the answer to your question. ASLR takes away between 0 and 2048 bytes of your initial stack randomly.