Inaccurate data usage on Xfinity Mobile

For the last several months have had a hard time getting data usage under control.  Seemed that no matter how many times I reminded my family to turn off cellular we kept going over on our data usage.

On 8/9 at 20:25 I got email that I had reached 80% of data usage (3 GB shared) for the plan.  80% of 3 GB means there is .6 GB remaining.

Have 3 lines that use data.  4th is used by elderly mother that doesn't ever use any -- that line sits at .01 GB every month.

Reset statistics on each of the 3 relevant phones and disabled cellular data for each application and then on the phone itself.

Users do turn on data for a few seconds if need to text or something like that, but then they turn it back off.

On 8/17 at 18:45 I got email saying had used 100% of data.  Sure enough online showed today I have used a total of 3.01 GB

I'm thinking, finally, I'll have proof of who is sneaking data.

I check the 3 phones cellular data usage and see one at 236 MB, one at 55 MB, and one at 66 MB, respectively.

That totals 357 MB.  Rounding up it is 400 MB or .4 GB.  So my phones say they've used .4 GB yet I'm billed for .6 GB.  That's a 50% differential!!!  

Spent 90 minutes on phone and tried to no avail to get daily usage on the phones.  Kept getting usage is only available for the month.

Only conclusion seems that Xfinity is padding the data usage.  Couple other threads on the forum seem to indicate that same conclusion.  I could understand ~200 MB inconsistently between phone usage stats and what billed over the course of a month, but not 200 MB over the course of 8 days.

What will it take to get to the bottom of this?  Am I really going to need to reset data stats on each phone at the end of each billing cycle to prove how much data we used and get an appropriate refund?

As an humorous aside, I just got a text about rating my recent phone interaction (the one I describe above).  When I responded 0 -- Not at all likely to recommend Xfinity, the automated response I got back was "We are not able to handle your request"