Mine does the same, I figured its normal.

 

When you have an incoming call and the screen is locked, you can just push the green call key to answer. You don't have to unlock your phone to answer.

 

I wonder if it's different because you have pattern lock on your phone. I haven't turned that on, and my phone answers with just a swipe (or the green button).
I'd GUESS that because you've got a pattern lock, the phone won't unlock without the pattern (just like you told it to... )

 

.. I have the pattern lock and can still press the green button to answer the incoming call when the phone is locked