I think it's a case of how big a difference does a single point make?
Marines are supposed to be able to basically tear humans apart like wet paper, but they're only 1 pt of Strength higher than guardsmen, so the gap between S3 and S4 represents a lot. Likewise, S3 represents everything from 'weedy conscript who can't do 10 press ups' all the way to 'unbelievably ripped Catachan who bench presses 400kg on his rest days'.
I don't know whether I'd imagine sisters as being somewhere between marines and guard in speed. I always saw them more towards the human end of the spectrum than the utterly transhuman marines.
They don't have the enhanced bodies of marines, so any strength enhancement is going to be purely the power armour, and if it's capable of boosting strength that much, it kinda makes the marine's genetic enhancements less important.