Yes but then it's also being developed by far more companies than Apple.
For 2012 Google submitted roughly twice as much code to Webkit as Apple (which came second), apart from them we have smaller yet notable contributors like RIM and Nokia (7% and 5% respectively).
Let's not forget that reduced maintenance costs are often touted as a reason to opensource code in the first place. And when you consider it was developed by Apple and so presumably fit there needs well when released it as working code, it's not surprising that adapting it to new use cases involves more work than maintaining it.
For 2012 Google submitted roughly twice as much code to Webkit as Apple (which came second), apart from them we have smaller yet notable contributors like RIM and Nokia (7% and 5% respectively).