Data

Almost certainly something akin to what others have described. You’re probably having background updates/syncing using data. You can view detailed data usage by device if you’ve got the apps installed. You can also limit the data they can use. Without a detailed list of the type of devices you use / services you have / etc that is about the most anyone can explain. If you’re able, ask them about changing service to an unlimited data plan. My internet is very reasonable for price and it’s unlimited. Not sure it’s kosher to talk about pricing on these forums but really, just ask they are willing to help if you’re a customer that pays on time every time and courteous to the reps.