probably because the calculators just use a generic estimate when figuring it into the mortgage payment, not the ACTUAL amount. the $1649 would be more accurate so just swap out the figures.
Better yet, give me a call or email me and I can work up some more accurate numbers for you. Online mortgage calculators can be helpful, but they aren't always that accurate because most of the time they are figuring at least a 20% down payment and the rates are a generic average as opposed to real-time rates.
It's always best to speak to a professional because you can't trust everything you read on the internet!