PAE was turned on but the licensing control code made by Microsoft checked if it's a consumer version of the OS or if it's a server, and if it's a server which version, and based on this decided how many memory to give to 32-bit OS. Consumer versions remained limited to 4 GB. This is detailed in the analysis of Geoff Chappell:

http://geoffchappell.com/notes/windows/license/memory.htm