Most coaches will tell you that the last lap feels slower than it really is. Sometimes it feels as though you could get off the bike and walk to the finish line faster! Don't give up or give in. You are going faster than you think you are.
And based on my power files, it's true, you probably aren't adding any significant power to the pedals. You are simply getting your legs out of the way of the cranks so that you can maintain the speed that you have already created.