I think it has to do with the data rate you can get with GMSK, which is the modulation used in GSM.
With a bandwidth of 200kz and GMSK modulation, you can get a gross bitrate of 270kb/s. Into how many “slots” can you divide this so that you can still have effective communication? Well, according to the people who designed GSM,the optimum is 8.
270/8 gives 33.75kb/s, and considering that a full rate speech coder requires 13kb/s, you have more than enough overhead to add protection/coding bits.
but you should be convinced that this is a useless question 🙂
so, in this regard, your answer is probably the best that can be given 😀
on top of what you said, if you add “idle” time (guard periods, Training Seq Codes) you’ll see that each TS has a throughput of 22 kb/s. Which is indeed just enough to get a good codec (old FR = 13kb/s) + a protection that keeps the rxqual < 4 when C/I = 9dB.
Now, why is the bandwidth 200kHz ? 😀 (another very useless question) 😉
Friendly Cheers !