Actually also with a 64 bit address space, you can easily get the same address if the allocator returns a cached block.

The problem with sbrk() is that if you allocate a few megabytes of memory + a tiny chunk, even after freeing everything apart from the tiny chunk near the "program break", nothing will be given back to the OS. If you use mmap() instead though, all pages can be given back apart from the one where the tiny chunk resides in. This makes for a tremendous difference sometimes.