Directors NI

I am aware of the cumulative nature of directors NI but i can't work out why, when a director leaves at the end of June, his month 3 pay should have more employees NI deducted as the cumulative method should mean less. I operated his pay on the alternative method so that his NI was flattened out through the year and knew this would change in month 3 when he left but I would expect a small drop rather than an increase. His gross pay has been £6,500 pm. I need to be able to epxlain it to him but I can't explain it to myself!