The page https://stackexchange.com/leagues/69/year/math/2020-01-01/796779#796779 contains infomation about the rep number of me ( user JCAA). It says that the total reputation is ~$12200$ and the reputation this year is ~$11700$. How can that be if I was present on MSE only for four months?
Some explanation when something like this happens is given in this post on Mathematics Stack Exchange: Why is my total reputation less than my monthly reputation? (You might be also interested in other posts linked there, for example: What are private reputation events?)
I will quote the answer posted there:
The discrepancy is by design:
- Total reputation is your current reputation on the site (accurate to within 24 hours).
- Month reputation is the amount by which your reputation changed that month, excluding "private" reputation events. Certain reputation events are excluded from your public user profile, such as deleted posts or downvotes, so those are excluded from the leagues as well.
In other words, "total reputation" includes private reputation events, while "month reputation" does not.
I do not really have possibility to check whether this might be the cause in your case. Since you have 10k+ reputation, you can check which of your posts are deleted by searching for deleted:1. For more details on finding your own deleted posts see: Is there any way to see my deleted questions or answers?
Downvotes are not likely to make a big difference. (Since downvoting an answer costs only one reputation point and downvoting a question does not influence reputation.) Still, you can find your downvotes in the votes tab your profile. (However, this shows both downvotes on questions and on answers, and for 10k+ users includes deleted posts. So not all entries you see there actually cost you some reputation.)
Disclaimer: I have posted this answer based on the OP's request in a comment. (Although the same thing was already explained in the previous comments, my addition was the I included also the relevant links.) I cannot tell whether or not this explains the discrepancy in this user's case. (Only the diamond-bearing users and the user themselves can check whether they have enough deleted posts to make this difference. And the downvotes are visible only to the user - not even to the moderators.)