QCCowboy is spot on- this is an app thing (iOS actually works the same as Android). Thanks QCC for the assist!
By default, any thread you post or respond to is subscribed. Anytime that anyone responds after that, it gets flagged as a subscribed thread and that number goes up.
I found a way to change it to "no subscription" because I had a crap-ton and it stopped being useful.
To clear them, you have to go into the thread and "read" it. Bear in mind, when someone responds- it'll come back unless you unsubscribe from it.
If you want me to find out how I changed my default to not subscribe, I can.