Coalition officials in Kabul told Khaama Press the insurgency was not in steep decline last year and there was no drop-off in the number of attacks by insurgents.
Coalition spokesman Jamie Graybeal said the error does not change officials' basic assessment of the war in Afghanistan.
A reduced threat from Taliban forces and the ability of local Afghan fighters to take over has been cited as the main reasons for withdrawing U.S. combat troops from the country in 2014.
A spokesman for U.S. Defense Secretary Leon Panetta said it was disturbing to learn of the error.
"This particular set of metrics doesn't tell the full story of progress against the Taliban, of course, but it's unhelpful to have inaccurate information in our systems," spokesman George Little said.