I've wondered about this because I would think that the day the title is signed over and the new owner signs it, then all tax responsibility is on new owner. I've bought my fair share of non-drivable junkers before and waited years to do an official title and paid from that date all the way back to the date on the title when I bought it . Then there's been times where the amount due was substantially less that what I was expecting but I have also lived in 4 different counties in the last 10 years so there's extra variables there .I would guess either the buyer hasn't transferred the title yet or your tax bill was created before the transfer.