3 identical loops round my local loop. The first 2 are 3 days apart, the times are within 10 seconds, but the 2nd loop was averaging approx 7 heart beats per min more - even though the track was dryer... why?

(unfortunately I didnt take the GPS with me for either of these runs so no gps track log - I'd sent it back for a replacement unit)

The green track is from a few weeks later (HR recorded on a different HR device so looks a bit different but is comparable) -  I just cant beat the 1st time (though it was pitch black when I finished this loop (I have a good light), and 4 degrees C), but look at the heart rate, its way up there!