Basically DR works like this: Every application within 60s from the previous reduces the duration by half. So you'll see 100%, 50%, 25%, resisted. In 2.0 the DR timer was based on the FIRST application of stun, now in 2.1 its based on each successful application meaning when that last 25% stun lands your 60s starts over necessitating a 60s gap between the last and first "fresh" stun. If that makes sense. To my understanding fully resisted stuns do not effect the DR timer applying multiple stuns at once also does not.

If you only got off two stuns then he went immune then someone else had stunned him. Most likely a DRG/MNK/WAR using their stun for dmg as is probably habitual.