Question about Frame Data/Attacks

So I guess I’m kind of a noob. I just started playing Sf seriously when SF5 came out. Anyway I had a question I think is about frame data or at least attacks. How come sometimes I’ll play say an Alex, and I can poke him and stuff moves he’s trying to do, then another time I’ll play another Alex and everything he does beats what I’m trying to do. Like his LP beats my LP , etc. But another Alex it will be the reverse. Does that have to do with lag in the game? Just the game is “seeing” my inputs after his? What gives? Also sometimes say Birdie’s EX bull rush will blow through anything my opponent is trying to do and other times it gets interrupted like a mofo.

The answer to whether a move should win in any scenario comes down to frame data and hitboxes. Player knowledge, reactions and lag are all factors that play into the actual outcome in online play.

What you choose to do should be based on the frame data, otherwise your play will be based on tactics that good players will easily counter, especially offline where lag isn’t a factor. If something you do unexpectedly loses or gets punished, take a look at the frame data/hitboxes involved and adjust as needed.