First of all, I want to make it clear that my internet connection is through a shared VPN. Many different people get access to internet using a same VPN server. This might be the reason for the following issue, but I am still skeptical about it.
Several weeks ago, I created my Stack Overflow account using Google login. Today I realized that I have a question that should be asked on Data Science, so I opened datascience.stackexchange and was trying to join the community. It told me that I was about to create my account using a login from Google, which I am not surprised about, because my Stack Overflow account is still logged in (although my Google has logged out).
So I just clicked the button, and then I was shocked:
Instead of the common random pattern avatar, it was a photo of a man. Moreover, this account has a name V.J. (abbreviated to protect his privacy, as for now I am still not sure whether he is a hacker or a victim), which is definitely not my name. I also noticed that this account was created 3 months ago, which is ridiculous (I have just joined Stack Overflow for no more than 1 month ago, and never used any other Stack Exchange site except for this question).
I then wanted to change the avatar and the name, because it's not me! So I clicked "edit profile & settings". And then I had a new finding:
When I clicked "Change picture", instead of two choices between Identicon and Google (remember, I created my account using a Google account), it was Identicon and Facebook!
This is indicating that, this existing datascience.stackexchange account was created by Mr. V.J. 3 months ago using his Facebook account. Then why would Stack Exchange bind this account with my Stack Overflow account?
I can confirm that no one else has used my computer to log into Stack Overflow, and I never share my Google account with anyone else.
Interestingly, when I tried to do the same (use login from Google) to create an account on Meta Stack Exchange, it seems that everything worked as expected: I got a random avatar by default, instead of Mr. V.J.'s photo, and now I am using this account to ask this question.
Why would this happen?