It is because it is the timing load with the most populated SRTs, nothing more nothing less. There are specific OTM timing loads but I dont know if these feed down to RealtimeTrains in the same way. Some timing loads dont such as Chiltern 168s showing as 158s.
In the CIF there is literally only a power type and timing load field. The speed is also derived from these. There is a translation mapping between the ITPS versions of these and the CIF version, but for quite a few they are so numerous that it's not worth even trying to decode it. (A future version of RTT does have a decode table for some.)