My understanding is that this is done so people get consistent monthly payments in the common case. Otherwise people who plan based on their monthly pay at the

My understanding is that this is done so people get consistent monthly payments in the common case. Otherwise people who plan based on their monthly pay at the start of the year might end up in serious difficulty when it suddenly drops. If this results in your being underpaid for whatever reason, it will be corrected at the end of the financial year.