The augment maṭ comes before ḍaṭ in the sense of 'making full this number' after a Numeral which ends in n when no other Numeral precedes it.,
Thus पंचमः 'fifth' (the completer of the five), सप्तमः ॥ Why do we say 'ending in म्'? Observe विंशतेः पूरणो = विंशः 'twentieth'. Why do we say 'not preceded by a Numeral'? Observe एकादशानां पूरणं = एकादशः ॥,