Negative Heart Rate

I am just getting to downloading and updating my training log from my trip last week to Aberdeen.    There is one thing that really stands out.  My heart rate goes from around 170 bpm when I started  and continually drops to around 156 bpm when I finished.  I did start faster than I finished by about a minute per mile.  I never thought it would make that much of a difference but I guess it does.

I looked back at the run on Monday and about the same thing happened but not at the same scale.  Interesting