Welcome to our Community!
I definitely get your concern and I assure you that our system only tracks usage that was made under our network. I'll need to ask a few questions to make sure we get to the bottom of this:
1. What phone model are you using?
2. If you're using an iPhone, is the WiFi Assist option active? You can check that under Settings - Cellular (at the bottom of the page)
3. When you weren't connected on WiFi, how exactly did you use your data, what type of usage was done?
Let us know.