For the past year or so, Comcast has been experimenting in select markets with a new data cap scheme where users get charged overage fees if they use more than an allotted amount of data each month.

These data caps have generated swarms of complaints to the Federal Communications Commission, as many users hate the idea of seeing their bills increase if they watch one too many Netflix shows.

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And that’s not the worst part of it, as a detailed report from Ars Technica shows that Comcast isn’t correctly measuring data usage for many customers, and is thus sending them monthly bills with wrongfully inflated charges and fees.

Ars Technica‘s investigation started when one customer told the publication that he received a bill that tacked on a whopping $570 in charges over the span of just one month for data overage fees. The customer and his girlfriend both work long hours and couldn’t possibly use all the data that Comcast claimed, simply because they weren’t in their apartment long enough to rack up such big charges.

Comcast tried to blame the customer’s Apple TV set-top box for eating up data while he was outside the house, but an investigation into the box by Apple showed that it was not running unauthorized downloads 24 hours a day.

So what’s really going on here? Ars Technica tried to get Comcast to give detailed breakdowns of how it monitors customers’ data but the company refused to do so, while at the same time insisting that its data measurement tools worked perfectly.

Comcast uses a consulting firm called NetForecast to help it confirm the accuracy of its data measurements, but the company only monitors 55 different accounts. It would be too expensive for the company to put monitoring equipment in the homes of its 23 million other customers, so it’s simply relying on a tiny sample to show that its measurements are correct.

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In the end, Comcast has still offered no explanation for why customers are getting hit with higher bills and it won’t give them detailed breakdowns showing exactly what is purportedly causing their accounts to use up so much data.

So if you receive a shockingly high bill from Comcast on a given month and you want to challenge it, don’t expect that the company will go out of its way to help you figure out what’s going on.