On the main Stack Exchange sites that I use, I post under my own name. However, on the Academia site, which I post on infrequently, I am assigned just an anonymous user number. This is no big problem for me; but I am just curious about why it is so.
Your Academia profile was originally registered as a separate account with a different email. Back in 2014, you did something that caused that account to get auto-merged with your current account (there's not a reason recorded there, so I can't tell you exactly what). Merging doesn't inherently trigger any sort of profile update, and it appears you just haven't edited your profile since that merge occurred, so it's remained in that state since the merge.
The Academia ♦ moderators will probably be able to see who changed your username and when, but you can easily change it on this page: https://academia.stackexchange.com/users/edit/10691