You really ought to study a bit more about the history of various operating system developments, before you make uninformed statements like that.
The internals of OS-X are largely BSD Unix. Modified, yes; and coated with a slick GUI; but by and large, BSD Unix, legally licensed.
The internals of Windows are largely the Win-NT codebase developed by Dave Cutler of DEC in the early 90's, are heavily modeled on DEC VAX/VMS, and like VMS, Win-NT borrowed many internal ideas from Unix. You may also recall that Microsoft was originally a Unix outfit prior to them buying MS-DOS, and the system calls of all their operating systems have incorporated large amounts of Unix system-level functionality and compatibility.
I'm not saying Windows internals are mostly Unix, the way OS-X internals are (Windows would be greatly improved by that, IMO). Just saying, there's more commonality than you let on to.
Actually dayglored, you're forgetting the little tie-up between Microsoft and IBM working on OS/2 before Microsoft had learned enough about multi-threaded, multitasking, multi-user OS principles from the masters at the time - IBM - then split off to develop NT. Then they hired Dave and somehow kept their shirt after IBM let them off the hook for pulling out of the joint venture (IBM still believed they could rule the PC OS market at the time since they made a good penny on it the first time and were sorry they let Microsoft get their foot in the door).