Previous allegations that AT&T has charged users multiple times the actual data usage have come back in spotlight even as the carrier is bracing to face ire from users over its plan to throttle data speeds for high-usage customers.

Some analysts are already asking if AT&T is entirely honest in its estimates of the number of people who will be affected by the data throttling plan, which will be effective on Oct. 1.

"Either AT&T is exaggerating the impact of this five percent and making a spectacle out of the policy change as a political move to justify the case for why it "needs" the T-Mobile acquisition approved, or AT&T is not being completely honest with regard to how many users will be affected or what the impact will be," wrote Tony Bradley in PCWorld.

In January, a lawsuit had been filed against AT&T in US District Court in Northern California, which claimed that the carrier's billing system recorded data use up to three times the actual use.

See what Ars Technica has reported about this class action suit in the past: "The independent tests were conducted with multiple devices over a period of four months, and allegedly show that AT&T's billing system regularly recorded data use that exceeded actual use by 7-14 percent, on average. In some cases, AT&T reportedly recorded data use as much as 300 percent of the actual use."

According to the lawsuit, AT&T's billing system was inaccurately recording the date and time of data usage. The carrier has consistently denied the truth of these allegations, saying they were "without merit."

There has also been confusion about another billing practice at AT&T, which gave rise to allegations that the carrier was forcing phantom charges on customers, meaning that users were being charged for data downloads at times when the devices were not in use. However, AT&T explained that this only related to the technical setting under which it captured data use records nightly.

Earlier, AT&T said it will reduce the data browsing speeds for the top five percent of its heaviest users on unlimited wireless data plans from October 1, in response to the explosive data usage growth and network congestion.

The carrier had assured that But limiting of data speeds will not affect the vast majority of customers who are either on tiered plans or moderate data users on the unlimited plan.

But those who use unusually large amounts of data will be affected. "These customers on average use 12 times more data than the average of all other smartphone data customers," AT&T said.