I don't think that this is a legal question. If your mortgage has an escrow required, you will go back to the original documents to determine what they require. I suspect that if your escrow number is smaller, it is because there was an escrow over payment the previous year. If your tax bill changes, that can also change the escrow amount. Did the tax bill go down because of a lowered assessment? Or because of a new Personal Residence Exemption?
Why don't you just ask your lender?
THIS ANSWER IS PURELY FOR ACADEMIC DISCUSSION ONLY AND DOES NOT CONSTITUTE ANY TYPE OF LEGAL ADVICE OR LEGAL REPRESENTATION.