And there goes any credibility I had at being a mathematical genius.

I think it's because I used two different things to determine pace and time. The thing I used to track my time was my GPS, it doesn't collect speed or height, it just tells me what turn is next and how long I've been going. I paused it eventually at the stops, but not without a few minutes extra rolling by. My speed/pace was determined by a different app and I never bothered to even switch to it, since it is set to tell me my current speed at a given interval.

It never even crossed my mind to consider that, and I feel even more stupid because I kept the speed/pace thing going during my cool down, which is probably why it's so much lower than 18.