As the MAS-SBC run is at 61kmph, it has been termed an SF.
Earlier many long distance trains with such schedules (SF in one section and non-SF/passenger in another section) used to have different fare structures (Like Howrah Kalka mail as an SF between HWH-NDLS and a normal express after that, or Marathwada express which is termed as a 'Composite' train).
For very long distance trains, such an arrangement was acceptable.
But...
more... in the recent years, many trains going to major cities like SBC/SC etc had to be extended to some nearby places purely due to rake parking constraints at the big terminals. In such cases, due to traffic considerations on the section between the major city and the train's terminal, it is not always possible to maintain a higher average speed in the extended run.
In these cases, the train would run as an SF in the main passenger heavy portion of the route, with little traffic in the extended portion. For these cases, it would be a loss to the railways to let people utilize a modified fare structure as most of the passengers would be traveling in the SF section of the route. So for such cases, IR is charging SF fares throughout.
For example, in the recent extension of the MYS-YPR SF to YNK, the train is no longer an SF if the end-to-end run is considered. But it runs with an SF schedule till SBC, which is the major passenger drop-off point. Letting passengers book till YNK as a normal express train and get off at SBC will be a loss for the railways. So they charge SF fares for the end-to-end journey.
In the same way, 12691/92 is mainly used by SBC passengers only. There is hardly any end-to-end traffic on the train, and it also does not have intermediate stops between SBC-SSPN that passengers will use. So it is treated as an SF end-to-end.