Agree, it has nothing to do with HR, or even speed or sex.

From a purely physics point of view, the amount of energy required to move a certain mass a certain distance is the same irrespective of speed ( I know that doesn't sound right, you intuitively think you must burn more energy if you go faster, but you don't. The effort is more as you are burning that energy over a shorted time period, but actual energy is the same ).

If it is showing a lower figure for the same distance then your weight setting in the watch must have reduced.