After a 2.5 hrs ride, 64 km, mean speed 26,7 km/h my Garmin 130 gives 84 calories as burn on this ride. On a similar ride before it gives 600-800 calories. 84 is way too low, what could be wrong in the settings? Strange thing, I did not change anything. And this is already second time it gave a very low calorie rate. It looks like sometimes the amount is Ok, other times it does not register?