How are people rated 800 when they once reached 1800?

Can you lose your chess knowledge/progress?

Today I kept playing against people ranked about the same as me, but they all played noticeably better. So I checked their stats..
Their maximum reached ranking in the categorie almost everytime was way higher than they had at the moment. Not 100 or 200 but more like 500-1000...
i.e. I played a guy ranked about 800, while his maximum reached was 1800..
Do they let themselves drop in ranking to play weaker oponents, or what s happening there?
Kinda annoying, if I want to get beaten I ll play an AI.

They may have been very good at chess, or they made the account as advanced then they dropped in rating

you don t simply drop 600-800 in rating.
Maybe they used chess engine, and they played without it and dropped.

But I guess I ll never find out

https://www.chess.com/member/schrapfi 
this guy for example, 15/10 chess,  highest rating 1400, now at 800
just seems strange to me

They’re ratings were high when they first opened the account...and dropped almost immediately 

Look at his stats and go through the timeline

In chess.com 200 rated beginner can open an account as 2000.

If you take a look at his stats, and for rapid (15/10), you make the graph show all time, you will see that he started off at 1400. That is the default rating that the website gives. Like @jtmccann15 said, you can see that the rating dropped almost immediately and stabilized at 800

And an eventual 2000+ player can start out as 800 on this site if he feels that his chess is "not good enough" relative to the playing field at the time of signing up.

oh that explains it, thx a lot