It happens to me also. Some calls I see the decline option or accept option. Others I can only slide to accept or I have to let it continue ringing and ignore it. I can't remember but does pressing lock button also ignore it?
[Raises Hand] I know this! Every morning my wife calls me at 5:30 am, and I always get the "Slide to Answer" option. If the phone is locked, you get the slide. If unlocked you get the accept/decline. Someone said if you swipe up on the slide to answer, it will show the option to decline. I have not tested that. Also they say 2 presses of the power/sleep button will send the call to VM.