Have you considered diet and the weather? If you have had alot of sodium or caffeine, your heart rate will be higher obviously. When it is cold, I often overdress (rather be hot and sweaty than cold personally)and when i am sweating the cold, it seems to stress my heart more. If you are dehydrated, that could do something with it as well.
Do you ride them at different times of the day as well? How rested your body could have something to do with it I guess.
I just started HR training about a month and half ago, so I still dont have a complete handle on what effects me and what doesnt, but so far, it seems sodium, caffeine, water intake, and sleep can cause major differences.