Data usage doubles with data cap over unlimited

This has been going on for over a year (of my 20 with Comcast). Data usage gets close to the TB limit so i add unlimited. So I dont have to think about it, I keep unlimited until i see that for each of the unlimited months, data is only 50%-75% of what it shows on capped months. Actual household usage is basically the same. I call and get the unlimited removed. Then, bam, next month usage is reported as much higher again. So far,  I've not exceeded the cap; however,  it appears I may today. I run mesh networks so can't get specific  reports on usage from my router, but I've never found rogue users on my networks. If someone had been using my data,  it would not have stopped on unlimited months. If the data usage showed relatively the same on capped months vs uncapped months, I wouldn't question what's going on.  If we (3 people--one who never streams anything and two who stream lightly --based on info provided by Comcast on data loads) really use more than 1.2TB a month, I'd just keep unlimited.  It seems as though capped data usage is somehow metered differently from uncapped data. I've read a closed forum on sudden high usage reports with no resolutions posted except for one who discovered it was an "i" (Apple) running in the background.  None of those posts dealt with usage discrepancies between capped vs uncapped months. Is there anyway to check out how these discrepancies exist?