I'm going to chime in here since everyone downvoted the OP. My guess is he got the Gmail account early. This was a known bug early with Gmail. They fixed the signups, but it seems there were some issues that continued if you had already gotten an email address that collided prior to the"fix".

I created an account for my daughter that had this issue and it was really messed up; we can see emails sent and received by another person which included a bunch of personal information; job application conversations, flight details, etc...

We ended up just switching our daughter to a new address, but if we had a more malicious nature; this is a big security issue.

Indeed, I created my account when it was invite-based. That must be like more than ten years ago.

Thanks you for understanding.