I think its the excitement of the coyote showing up and the tunnel vision. Funny you mention this cause I've thought the same thing a few times. This week I had the same thing with actually getting a large male. I hunt in the dark, so I think that intensifies the effect.
Many times I guessed the yardage way short too, I always count it off as I walk to the carcass. It's of no ballistic concern because of the fairly small clearings for making shots.