It is fine, don’t worry. A call does indeed need a constant 12.2kb/s (useful) throughput. The actual throughput on the air of a TS is above 22kbps, but that’s including all the “radio” protection around the useful signal. You shouldn’t be counting it.
Just don’t forget that the timeslot has also a useful throughput capacity of 12.2kb/s and you have no reason to worry. I’m interested to know what you would do with such a figure ?
If you want to compute the throughput the voice is going to use in an IP transmission network, then you should also add the headers and the related signalling (about +0.9kb/s per call).