How is it possible that I can be there 1800 on chess.com Rapid but only 1700 lichess • page 1/1 • General Chess Discussion • lichess.org

I thought my rapid rating was inflated at first, but then I started consistently beating someone 2000 on lichess.

You just can't interchange ratings from different websites. They have different user bases possibly leading to different ratings even if the underlying rating system is the same. If I am informed correctly lichess and chess.com use slightly different rating systems (Glicko on chess.com and Glicko-2 on lichess), possibly adding to the divergence.
Additionally, ratings just approximate your playing strength, but aren't exact. In your profile on your rapid rating you'll see a raing deviation (currently 56 for you) . In statistical terms it is standard deviation, meaning that the rating system is 67% sure that your "true" rating is in the range +-56 around the rating it currently assinged to you, translating to 95% confidence of double the range, that is your rating +-112 currently. On chess.com your rating deviation will probably be similar, so also that alone can explain the rating difference.
From what I have read (https://chessgoals.com/rating-comparison/) lichess ratings tend to be higher than chess.com ratings because of the user base. However, seeing how much uncertainty is involved with ratings I still don't think this case of higher rating on chess.com to be very uncommon.

You just can't interchange ratings from different websites. They have different user bases possibly leading to different ratings even if the underlying rating system is the same. If I am informed correctly lichess and chess.com use slightly different rating systems (Glicko on chess.com and Glicko-2 on lichess), possibly adding to the divergence. Additionally, ratings just approximate your playing strength, but aren't exact. In your profile on your rapid rating you'll see a raing deviation (currently 56 for you) . In statistical terms it is standard deviation, meaning that the rating system is 67% sure that your "true" rating is in the range +-56 around the rating it currently assinged to you, translating to 95% confidence of double the range, that is your rating +-112 currently. On chess.com your rating deviation will probably be similar, so also that alone can explain the rating difference. From what I have read (https://chessgoals.com/rating-comparison/) lichess ratings tend to be higher than chess.com ratings because of the user base. However, seeing how much uncertainty is involved with ratings I still don't think this case of higher rating on chess.com to be very uncommon.