Technically, Lichess doesn't use Elo, but a variant called Glicko-2 (wiki link if you're interested in the gritty mathematical details). In addition to your rating, Glocko-2 also keeps track of its variance and volatility (although they are not publicly displayed). When you create an account, the volatility is high, meaning that you will gain and lose lots of rating at first. Once you have accumulated more games, your volatility will decrease and the rating will settle to reflect your performance.