Strange. It should be technically impossible for your phone to have different stats from us because the API we use to gather data allows us to gather it directly from your phone. That’s how GlassWire can have data from before when it was installed on your phone.
When I have seen this issue it has always been because a third party app somehow blocks the “phone” permission we need to access the data usage API.
Technical details of why we need that permission are here.
https://blog.glasswire.com/2017/04/18/glasswire-android-permissions/