It's most likely due to a difference in the actual and estimated property taxes for last year. If the estimated was more then the actual, the overpayment will reduce the amount you will pay this year.
Yes, look at your statement and you will see there was an overcharge for property taxes this year for which you are getting a credit in 2013. Disney must estimate those taxes annually without knowing what the local politicians will actually charge and in the past there have been both overcharges and undercharges (which leads to even higher dues than expected the next year).
The funny thing is when DVC undercharges, they make you pay the full annual difference in January. But if they overcharge, they spread the credit over the full year.
For my BWV 250 points, the tax credit is $16.13. My monthly dues went from $117 to $120.34. For 2012, I had to pay a $21.55 undercharge in January.
Just try to remember the reason why they are lower NEXT year when you get your assessment. It's easy to think "omg they went up SO much", when you were having a sort of a discount *this* year.
Dealt with that situation in 2012, because of overestimate in 2011. Had to keep reminding myself of the reality!
I'm not too worried about having to pay undercharges in January I normally pay the whole thing in January anyway. DH works at Target so we are going to do the gift card thing this year which will get us quite a discount on our dues.