it is difficult to make an assumption. What I can tell you for sur is that I don’t have an answer. But we can discuss 🙂
The BCCH TRX will be “filled” with TCH calls only once all the other TRXs are filled up.
Therefore, this is what is called “busy time”. A lot of traffic in the cell… It could last one hour, or just few seconds. I can happen many times during the day, that the cell is suddenly congested.
At such time, the radio conditions are rather poor, therefore MS which are nearer to the BTS have more chance in successfully establishing a cell. that could explain the lower TA on the BCCH TRX.
Now, another idea is that hopping TRX provides frequency diversity, which lead to a bigger coverage. Therefore a call established on a hopping TRX *could* be kept further away than a call on BCCH carrier. It is a little far-streched, I agree… :))