He means why does "-ний" decline into "-яя", "-ее", "-ие" etc. instead of "-ая", "ое", "ые" etc.

...to which I have no answer. I don't know. It may be worth noting that in most other adjectives ending in "-ий", the stem ends with a "к", "ш" or "щ", consonants for which the spelling rules require that they cannot be followed by the letter "ы".

(But I really don't know if this has anything to do with it...)