Fred Prouser/Reuters

Last year, AT&T warned smartphone customers with unlimited data plans that it may temporarily reduce their Internet connection speeds if they were in the “ top 5 percent ” of heaviest data users. Only recently have some customers begun receiving notifications about their data connections being slowed down, and AT&T claims that it isn’t as bad as it seems.

John Cozen, an AT&T customer, was among the first to report receiving a notification of being “throttled,” or having his cellular Internet connection slowed down until the next billing cycle. Over the weekend, he posted his e-mail conversation with AT&T, which informed him that he was in the top 5 percent of heaviest data users because he had used 2.1 gigabytes of data.

Mr. Cozen, who said he has an unlimited data plan, believed it was unfair to be throttled. He pointed out that the unlimited data plan costs $30, and AT&T offers a limited data plan for $30 per month that offers 3 gigabytes of data. So what gives?

In a phone interview, Mark Siegel, an AT&T spokesman, said that as of last summer, the top 5 percent of AT&T’s heaviest data users have typically used 2 gigabytes or more per month.

Mr. Siegel said that even if you do exceed 2 gigabytes of data usage and qualify as one of the top 5 percent, that doesn’t absolutely mean you’re going to be throttled. AT&T will only reduce speeds for the top 5 percent of users in areas where network capacity or spectrum is insufficient, he said. In other words, throttling is done on a case-by-case basis, not based on a hard number, according to AT&T’s claims.

“There’s a very good chance you wouldn’t be slowed,” Mr. Siegel said. He added that in the last month, less than 1 percent of AT&T smartphone customers were affected by the policy.

AT&T’s throttling policy only applies to customers with unlimited data plans. The wireless company stopped offering an unlimited data plan in 2010; those who had already signed up for one before the cut-off had the option of being grandfathered in.