DMV1200 Detects 33 voice boards instead of 30 , what are extra boards and channels ?

I have a DMV1200 board, I get Voice Board counts with   sr_getboardcnt("Voice", &BoardCount); 

It returns 33 voice boards , with total of 132 voice channels.

as DMV1200 is 120 channel , I expect to open 120 voice channels so what are extra 12 channels for?

Should I open first 120 voice channels ? How and when should I use extra channels ?