I signed up with Pokemon GO back when it first came out using my Gmail account rather than a trainer account. However, I do also have a Pokemon Trainer Club account that shares a username with my Pokemon Go account and also is tied to the same email I used to create my Pokemon GO profile.
When I log in to Pokemon GO, it shows my username (the same username that my Trainer Club account uses) and that it's linked to my Gmail address. Yet when I log in using the Trainer Club option, it's an entirely separate account.
Why does this happen and is there a way to be able to log in using either my Trainer Club account or my Gmail address to log in to the same account (since both accounts have the same username & same email tied to them)?