ARINC have the Eurocontrol contract to monitor altitude maintenance; someone has to sit on your jumpseat with a very state of the art GPS system and make regular logs of which autoiplot is engaged and what the altimetry is doing vs what the GPS says. You then get the tick if it is within limits on all autpilts and altimeters. If one particular airline or type doesn't meet the criteria then no approval until it is fixed.