The only scenario I can imagine that making sense in is if you are getting SSI or SSDI and the overtime you exceed the threshold to keep receiving benefits, but would not exceed the actual SSI benefit itself.

Or for SNAP or TANF or something.

... the only other scenario I can think of is your ex is commiting tax fraud.

... or they can't understand middle school level math... =[

That can happen, depending on your company's payment software. One place I worked at would calculate the expected tax burden based on the highest paycheck you made, so if you worked 50% overtime (thus making 175% of your normal paycheck), it would be placing you in a way higher tax bracket for the rest of the year, and you'd have to reclaim it all in the refund when you did your taxes.