Halevai
“Why bother fixing six hours to the same length when it doesn’t reflect reality.”
the answer to your question is to get two halchos to fit namely that say sof zeman shacharis is both after 6 hours and at chatzos. Yet the reality is that these arent the same times if you diefine shaos zemanios as 1/12 of neitz to shkiyah. So R’ Moshe “redefines” shaos zemanios as 1/6th of neitz to chatzos for AM shaos zemaniyos and 1/th of chatzos to shkiyah for PM shaos zemanios.
(At least this is how I recall the Tehsuva)