Anyone who has ever suspected that carriers overcharge for monthly wireless data can now feel some vindication: A new study from UCLA suggests they’re probably right. MIT’s Technology Review says that the study looked at mobile data calculations from two major wireless carriers and found that while carriers usually got things right, they “tended to overcount” when they did err in their estimates. And this doesn’t just affect people who spend their entire days streaming video either, as the researchers found that “even typical use of a phone could lead the data to be overcounted by 5 to 7 percent.”

The major reason for all the overcounting is that carriers calculate data based on when it leaves their network and not whether it actually arrives at users’ smartphones. So a user trying to stream video in an area with poor coverage might experience major packet loss during the video but will still get charged for that lost data since the carrier records it as having been transmitted. Technology Review notes that this issue particularly “affects video and audio streaming apps in particular because they use protocols that don’t require the receiving device to acknowledge the receipt of every chunk of data, as Web browsers or many other apps do.”

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