We don't account for alignment overhead...

"The pointer returned if the allocation succeeds is suitably aligned so that it may be assigned to a pointer to any type of object and then used to access such an object or an array of such objects in the space allocated (until the space is explicitly deallocated)."

In order to satisfy the above each pointer returned needs to be a multiple of 8 bytes in a 64 bit program. This should probably be tracked in the memOverhead stats?