nevermind..i found a doc with a clear explanation :)
Here is a summarized version in case you were interested;
The switch tags/maps voice to a specific queue. It then assigns a higher priority for queue 3.
Limit for shaping on queue 3 is selected so that it is 33 percent of the link speed.
This procedure ensures that the higher-priority queue does not starve other queues.
33 percent may sound like a lot but remember that is worse scenario and the transactions would have to be simultaneous.
The algorithm is based on "Cisco best practice."