The only two apps I have mobile data allowed for are Whatsapp and Signal. Now I am assuming that any data usage for them will be registered under them in mobile data. However, yesterday I reset my statistics and checked after an hour, and push notifications uses twice as much as either of them combined. I don't have anyother notifications enabled, and no other apps were open or have any logged data usage. Does this mean that, even though the data usage is related to those two apps, it gets logged under system services because incoming messages are related to notifications? This is the only explanation I could think of.