There's a big discrepancy between the number of wiretaps reported by the US courts and the number of wiretaps responded to by US phone companies.

Last month, the US Courts' Administrative Office said the number of wiretaps authorized in 2015, which allow the authorities real-time access to communications, stood at 4,148 wiretaps, up by 17 percent from a year ago. Not a single wiretap request was rejected during the year.

But that figure doesn't make sense when you look at how many government data demands were processed by the big telcos.

Verizon, AT&T, T-Mobile, and Sprint responded to 11,633 wiretaps during the year -- almost a threefold increase over the government's annual wiretap report. (T-Mobile alone said in its latest transparency report that it received hundreds more wiretaps than the government's official tally.)

And that's just the cell networks -- the difference is likely far larger when you account for landlines and internet companies.

So, how many wiretaps were authorized last year? Nobody can explain the discrepancy.

This isn't even the first time the numbers have come under scrutiny.

Albert Gidari, a former leading privacy lawyer who now serves as director of privacy at Stanford Law School's Center for Internet and Society, was first to notice a difference in the numbers. In the previous 2014 wiretap report, he noticed a twofold inconsistency between what the courts reported and what the cell giants reported.

In a blog post a year ago, he analyzed the numbers. Even taking into account the complexities of run-on and extended wiretaps, Gidari said the numbers still don't add up.

He told me on Tuesday -- a little over a year later -- he still can't figure it out.

"No one seems to have an adequate explanation," he said.

When reached for comment, a spokesperson for the Administrative Office explained, "numerous wiretap authorizations are not reported... in the year they expire because investigations are ongoing". The spokesperson also said it's possible that "one wiretap order can include multiple devices, therefore, the total number of devices tapped is likely to be greater than the number of orders issued". Also, if wiretap applications are granted but require an extension, the courts will not report the orders until after they expire.

That might apply to a few stray wiretap requests, but Gidari said that it wouldn't come close to explaining the threefold margin of error.

"It is inexplicable even considering that carriers may each have received an order that covered four different devices on four different carrier networks," he said. "But for that to explain it, every order would have to have at least three devices covered on three carrier networks to explain the numbers."

"Transparency is supposed to be about making it clearer, not more obscure or obtuse," he said.