I still don't understand why we don't go by the in-game timer like all the other metroid games.
To be honest I don't know either, but I'm guessing because it's hard to tell what gets cut off and where. Guess it's simpler to just use the total time like with the NES Metroid since that doesn't have a timer at all. Unless of course I don't know some different way times are calculated with the original as far as recorded/official runs go.