I was trying to join a Google Group for my university. I added my Yahoo email address and got an email on that Yahoo email saying "you requested to join a Google Group blah-blah-blah Join This Group" which has no problem since I requested it.
But when I click "Join This Group" button I was redirected to GoogleGroups page, there's a strange e-mail but it was on my name, I don't remember creating/using it ever. Instead of using my Yahoo email, it somehow found an email on my name and made that strange email subscribed to the group.
I don't remember that I created that strange email (eventhough it's on my name) and cookies etc. were not existed for sure. I clicked that email afterwards with clean browsers then again it uses that strange email.
How could that happen? Is there a security concern here?
Edit: Email address clarification: Let's say my name is John Doe. My Yahoo Email's format: firstname.lastname@example.org and that strange email is Gmail and its format is email@example.com