Why I use my data?

Maybe your phone is not always connected to the wifi? That happens to me sometimes, especially when I move around in the house. If the wifi signal is not string enough many phones will switch to network data. You can disable this manually in settings.

For some reason, it uses a little data every day for some people. You are not the only one.

It did the same for me but now I deactivated the data on my phone when I'm on wifi and it fixed the problem!

it's normal. System did use mobile data if you didn't turn OFF mobile data. You'd better go to setting and do it, if your data is limited.