My Thunderbird pulls in emails from my gmail account even when those emails have been deleted in my gmail account. I do not have them synced. How do I fix?

I go into my gmail account first and delete messages I don't care about. Then I will later use Thunderbird to pull in messages I want and be able to store them locally. I do not have TB and gmail synced. However, when I "get messages" in TB, it pulls in a whole bunch of emails that I had previously deleted from gmail. How do I prevent this from happening? Thank you.