United B763 near Indianapolis on Mar 7th 2014, disagreeing altimeter

A United Boeing 767-300, registration N668UA performing flight UA-970 from Houston,TX (USA) to London Heathrow,EN (UK), was enroute at FL330 about 50nm northeast of Indianapolis,IN (USA) when center control queried the crew to correct their secondary altimeter setting. The crew reported that their right hand altimeter was set correctly but was not within the 200 feet limit to agree with the other altimeters. The crew consulted with flight operations and subsequently decided to return to Houston. The aircraft descended to FL260 and landed safely back in Houston about 130 minutes later.

A passenger reported the crew announced flight computer inconsistencies.