About BW in RTP analisis, VoIP

HI, I am using ethereal to capture traffic RTP of VoIP, i have 2 PC,
and then tun ethereal on both, for example if i use the codec g711,
the BW (band wight) must be 80 kbps, but in one PC  can be fine, or
can be 81 kbps, 79 kbps. And if I use GSM, this must be 29 kbps, but
sometimes is 30 kbps, 31 kbps. I want to know is that is fine or is
there is something wrong. I want to know the why of this to explain to
the teacher.

Thanks..