I presume you are asking why sMP sometimes combos into 4HK and sometimes doesn't -
sMP has 21f hitstun, 4HK got 22f startup = can't combo normally
BUT if you hit an opponent right on the frame they are landing, there are 2f(?) of extra hitstun (for whatever reason), allowing the combo to work.
This is a bug that has been around for a while, but been deemed too unimportant to fix
So: This combo will work if you are frame perfect; else the sMP won't have sufficient hitstun to allow this combo.
Read: Don't do this combo.