Who I have the same questioned this topic

I'm a recent switcher from AT&T to Verizon. When my iPhone XR was in the monthly AT&T 9 GB plan, my data usage was always in the 6 GB range.  I have been a prepaid Verizon customer for a few months with the same phone, I've noticed Verizon calculate or accumulate data higher than AT&T.  I'm using fewer data and but it's reaching 15 GB per month with Verizon.      Has anyone noticed the data calculation difference between the two providers?