Property taxes and homeowner's insurance rates change yearly. It's impossible for the lender to proactively predict how much it will change. When the bill comes due, they pay it. If rates went up and the escrow account is short, they cover the shortfall, and adjust the escrow payment to make up for it. If people don't like it, then don't pay those bills via escrow on your mortgage. Pay those bills separately on your own. Then the mortgage payment won't change.

This has nothing to do with putting 20% down and avoiding PMI. PMI is completely separate from property tax/insurance escrow.