Wow, I would not have suspected `tee` is able to handle multiple processes writing to the same file. Doesn't seem to be mentioned on the man-page, either.
All tee does is write its standard input (a single file descriptor) to a file (a single one) and its own output. xargs is the thing running multiple processes (and they inherit the same standard output, your shell's).
What you're seeing is Linux being able to handle multiple processes writing to the same file.
Well, then that's a Linux feature I was unaware of. I found this SO[1] question with two conflicting answers that have almost the same number of votes, and even the "yes you can do this" answer seems to have enough caveats that it doesn't sound like a great idea.
