My real FIDE Elo was 2400 at max in 199x and in 2018 my real blitz rating was 17xx(Now I got few hundred points back in few tournaments). Not everybody who plays on platforms or even in tournaments have your "professional attitude" and cares about rating - I don't care about my - I play for fun, intentionally go for high risk combinations often loose to 1500-1600 players but at same time when I meet GMs I pretty often beat them or play on equals as that game for me makes sense and I'm concentrating and not playing fake combos just for fun. My online ratings are fluctuating like hell as I often play at work while in boring meetings and just resign when asked question and have to go or sometimes loose stupidly due to double focus! My own top rating on lichess is 23xx and often it falls to 22xx or even 20xx, but at the same time I have a 2400+ player against whom I lead 6:0, because I play those games - they make sense to work for me - if I play you I may easily loose as I don't care - I know I'm better player, nothing to prove.
So only 400 points diff between 2 online ratings and with different time controls - good but slow player. Or may be like me - does not care ratings!
Obviously it can be both - true reflection of players ability to play at certain speeds(lot of old people are just not good with mouse controls for example, so they loose fast games by quickly getting into time trouble) and it can be reflection of players true rating due to careless play or fun attitude!
Players like you who are analyzing his opponent ratings after the game instead of game itself never learn a game itself. If you will not change your attitude you will never learn to play chess!