I would agree that you are fitter than last year but the fact that you are slower at the same distance seems to cancel that option out. If you are fitter then: you should either complete the same distance with the same time but a lower HR -or- complete the same distance in a shorter time with the same HR. Make sense?

What pops into my head would be possibilities of overtraining - does that seem to be the case for you? Do you feel well -or- are you tired, lethargic, blah, bored, unmotivated?

Are you under more stress than normal? Are your eating habits the same? There are endless possibilities but maybe you can narrow it down to a few? Keep us posted!