Is the email in question something along the lines of firstnamelastname at gmail? I'm guessing your email address is a really common name that someone else keeps forgetting how their email actually deviates, or someone typos writing theirs.
Another possible scenario is that Gmail is getting wires crossed. I have had the account firstname.lastname@gmail.com for 20 years now. About 5 years ago, some dude in Australia (who coincidentally has the same rare last name as me) started using firstnamelastname@gmail.com. Based on the emails I've seen I believe that Gmail let him do this for a while, but eventually started delivering his emails into my inbox. I don't know if there was a technical change in Gmail for how they handled these addresses or what, but it's very odd.
firstname.lastname@gmail.com and firstnamelastname@gmail.com are the same address, according to gmail documentation. If this is what is actually happening (and there isn't a subtle typo, etc.), then something is more wrong than "wires crossed" & you should report it as a security vulnerability.
