This user competed in Round #623 in Division 2 and received rating change for it, even though he was 1900+ before participating and this round wasn't Div. 2 only. How was this allowed?

Update: I also noticed that there were several competitors (here, here, here, and here) who competed in the Div 1 version but were rated for Div 2. Again, I'm not sure how this was allowed.

Check out this guy's profile:

https://codeforces.net/profile/Daryusz

How did he get 55,000 friends?!

I've competed in several AtCoder Grand Contests (AGC) and have noticed that my performance in these is much worse than in Codeforces Div. 2 contests. Moreover, I find the problems in AGCs on average much more interesting than those in Div. 2 contests. I want to improve at AGCs specifically, and I was wondering if there were specific websites/sources of problems to train on that have the most overlap with AGC-style problems. So far, I've come up with the following:

-Practice previous AGCs

-AtCoder Regular Contest

-TopCoder (problems tend to be more math/thinking and less implementation heavy than Codeforces)

What are your thoughts on the above three sources, and do you know of any others that would serve as good practice for AGCs?