I wouldn't even attribute it to the whole "rubber arm" quality. I don't know how true that is of him any more. He used to be an absolute mule, and he can still eat up a lot of innings, but he definitely isn't the same as he used to be.
The reason I say the rain delay thing is because: 1. he's a seasoned veteran and cerebral pitcher, 2. he doesn't throw hard. Seems like a recipe for someone who could easily take the ball and toss it fairly well even after a rain delay.
Don't confuse lack of velocity with lack of effort. I can throw just as hard as Stephen Strasburg, but the ball doesn't have the same velocity.