Not quite...
Along with start up and recovery this game has the ability to cancel moves, thus changing advantage. I'm not sure if canceling occurs any time after hit, or it must reach the recovery frames, but we'll assume it's after hit and can be done in remaining active frames. By the way these are made up moves and frames (this example does not exist in the game as far as i know, but clues you into what occurs).
So if I have st.M 6-3-7 (+4) and st.H 12-3-15 (-8) it can play out like this going st.M to st.H:
st.M hit stun with our formula:
Enemy stun - remaining active frames - recovery frames = advantage
??? - 2 - 7 = 4
??? - 9 = 4
??? - 9 (+9) = 4 (+9)
???? = 13
Stun is 13 frames!
I hit at frame 7 with st.M, then press st.H at frame 8 of st.M, St.H begins the next frame (so would've been frame 9 of st.M), we now canceled st.M and began st.H while the enemy is in hit stun of st.M. so lets compare the hit stun of st.M and the start up of st.H
hit stun from st.M - start up of st.H
13 - 12 = 1 frame to land the move!
You must have perfect timing!, if you delay the cancel and do it a few frames into st.M recovery your st.H won't connect. This is why people drop combos, they cancel at the wrong time creating issues with hit stun finishing or hitboxes whiffing due to position and etc of the enemy from stun. So dropping combo's isn't random, you have intervals of time that it must be done in to work.