My husband and I purchased our first home and closed jun29,2007. Everything was escrowed?

our payment was 1,424.84. Saturday we recieved a letter from Amtrust Bank, annual escrow accouont disclosure statement, It says that this is not a bill but there is an escrow shortage and the new payment beginning on 12/01/07 will be 1,909.30--How can we have a shortage? Is our payment going up 500.00 a month, it says the shortage amount is 4,382.65. Can someone help me understand? Thanks