Video bitrate obtained > Video bitrate desired

[smegolas]

This is probably a silly question but I have never encountered it before.

I did a Xvid encode using MeGUI and the Xvid 2-pass HQ (No QPEL) preset. My desired bitrate was 1600kbps but the obtained video bitrate was 2200.

I always assumed a 2-pass bitrate targetting encode would give a predictable bitrate, surely that is the whole point. I did an encode right before using the same settings and I got exactly my desired bitrate.

So what have I done wrong? Is it preventable?

Thanks

[Selur]

1. use higher values in the overflow treatment configuration i.e. 10/10/10
2. don't restrict the quantizer range