As Bob said, these sites draw from all sorts of (sometimes errant) places, most the MLS, but the only place that appraisers, assessors, and other folks that matter will get their info will be the registry. THe registry has the correct info if it was correct on your paperwork.
One thought . . . is it possible that you had a concession on your purchase, such as the seller paying closing costs or the like? In some data fields in MLS and other data sites, this amount is deducted from the price to reflect the "true" price of the home. Depending on how this info was entered, this may account for the discrepancy. This is important info for area appraisers, and will appear in subsequent appraisals that utilize the sale of your home as a "comparable". Otherwise, someone may just have made an error. . . . one that will fall off eventually, with the correct info staying on record at the registry. So long as your paperwork reflects the correct price and loan amount, you are all set.
If it is really, really bugging you, you could email the agent - they can give you a quick answer as to how they reflected the information. . . . but as Bob said, I wouldn't worry about it.