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They use different rating systems, different starting values for elo and in general, elo is a measure of your strength relative to the player pool you're playing in, so your elo across websites is not meant to be comparable.

E.g. in Lichess you'll always start at 1500, while in Chess.com you can start at 400 or 1200 (and some values in between I believe), so it's pretty normal if your ratings differ with hundreds of points - in fact it would be very weird if they didn't (unless you're 2400+ I suppose).

By the way, an interesting comparison some people try to make is elo in Lichess/Chess.com and FIDE rating and see if one can estimate FIDE rating for someone who has never played over-the-board just from their Lichess blitz rating.