That’s an overly simplified version of what it does; any app with background app callouts will be polling in the background… but only after it has been launched in certain circumstances. If something messes up in the app logic, that issue gets cached to storage and recovered to working memory when background or foreground functionality is called. There are two ways to fix this: kill the app, or reboot the phone (or respring if you’re jailbroken). If you know some background app is misbehaving but you don’t know which one, you can either kill them all one by one, or reboot. Killing them all would be the more efficient solution.