The difference is that code from an existing developer is going to be maintained: that is, the existing developer will stay around to take care of it.

Code from an outside developer will just be committed and never touched again in many cases, since the outside developer doesn't intend to stay and work on the project. If you don't believe me, look at the history of the ffmpeg project: there are enormous swaths of unmaintained code (and uncommitted code) for exactly this reason.

Thus, it is often reasonable to have higher code standards for code whose author does not intend to maintain it into the future.