Most GPLv2 licensed software includes the clause "either version 2 of the License, or (at your option) any later version." Through this mechanism a lot of GPL licensed software becomes compatible with Apache 2.

Also, replying to something a little higher in this thread, I wouldn't say that Apache 2 defines all contributions as Apache 2. That section of the license starts with the words "5. Submission of Contributions. Unless You explicitly state otherwise, ..."

And so Apache 2 just becomes the assumed default license on contributions, but it's not at all forced or required that contributions come in under Apache 2.