AC3 Audio Question

[Rimmer209]

I have a question i would like to know from one of you really smart people.

I have some HD Files in MKV format, i thought i would experiment with converting them to HD DivX to see how they work out, i used MKVMerge to extract the AC3 file and thought i would use a muxing program to put it back with the DivX ( a solid idea at least )............However i know there are problems from my past tries, when i chcek the AC3 file extracted its 1hr 50 mins and 42 seconds but the original MKV file video and Audio is 1hr 51 Minutes and 12 Seconds.

The question is if the AC3 track was muxed ( and time synced ) to the video why is it now a different duration?

Sorry but its something thats bugging me

[Case]

Muxed video and audio don't need to have the same duration. It could be that the last 30 seconds are without audio. Or the first 5 seconds and the last 25 seconds. Those are the easy options. It could be more complex.