It's not your user.
Note that https://mathoverflow.net/users/56920/arnold-neumaier is a registered user who have no posts.
Whereas https://mathoverflow.net/users/56919/arnold-neumaier is an unregistered user which has a single post with four upvotes, and 41 points of reputation.
If you still have access to the unregistered account (through some browser window somewhere), you can register it using the same credentials you registered this account. The system will merge them and you'll have the points. If you don't, you can look here: https://mathoverflow.net/contact and use the option about merging accounts.