Different circumstances, basically.
Your rating will be different for a blitz game than for a longer one (some people have good ratings for longer games and some excel at faster ones).
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Because online chess is different from live chess. Live chess is when you play with someone that is online. For online chess, not necessairily. In addition to that, there are quick games, blitz games, and long games in live chess, while online chess does have times, but it is in intervals of days, so it is just 1 rating.
Why is there a rating for "online chess" and a seperate rating for "live chess"?