-Data usage scam

We had been hitting our cap, so I switched to unlimited, once we were on the unlimited plan, we were no where near hitting close to the old usage cap.  So since we have less income now due to Covid, I changed back to limited, since we were not getting any where near the limit amounts.  Changed last month starting this month.  Somehow, we are at our limit and going to have to pay more.  There has been nothing different this month from last month, except removing the unlimited plan.  Do they calculate differently depending on your plan, just so you have to pay more?