Passenger trains were operated by timetable authority on the SP until the 1980s. That meant they were given rights to occupy track by a schedule printed in the employee timetable. Absent any other instructions from the dispatcher, other trains had to be in the clear in advance of the trains published in the timetable.
Amtrak and SP tried to keep the "public" train number the same as the "railroad" train number. On the SP all trains were either eastward or westward from San Francisco, and "westward" (towards San Francisco) had odd numbers, "eastward" trains (away from San Francisco) had even numbers. Since the Coast Starlight went through the Bay Area, it changed timetable direction in Oakland, and so required two operating train numbers.
So Amtrak train 14 in the Amtrak public timetable was SP train 13 in the SP employee timetable from Los Angeles to Oakland, then SP train 14 from Oakland to Portland. Don't know what BNs numbers were. Conversely, Amtrak train 11 in the Amtrak public timetable was SP train 11 from Portland to Oakland, and SP train 12 from Oakland to Los Angeles.
Since the 1980s all trains are run the way "extras" were before, with no timetable authority and specific authority from the dispatcher, and no trains appear in the employee timetable any more.
Amtrak train numbers 11 & 14 are the anachronistic remnants of the old timetable authority. It is probably easier to leave them the way they are than to change them.
Superstition against "13" has nothing whatsoever to do with it in the Coast Starlight's case.
The SP Coast Daylight were SP trains 98 and 99. The SP Coast Daylight never ran with numbers 12/13 (or 11/14). If I recall correctly, the Cascade (Oakland-Portland) was 11/12, though.
Right after Amday, the Coast Daylight/Starlight (it's initial Amtrak name for the 3 days a week it ran through past Oakland) ran with the Coast Daylight's (98/99) and the Cascade's (11/12) train numbers. Amtrak "normalized" their train numbering that Fall and the Coast Daylight/Starlight, later the Coast Starlight, assumed their 11-12/13-14 numbers then.
The "Spirit of California" also ran past Oakland to Sacramento and had a similar "split" train numbers for the same reason (15/18? I don't remember, although I know they were in the "teens).
Note that the Capitols, which run past Oakland between San Jose and Sacramento do not follow this pattern but have "normal" train numbers. That is because that service as instituted after timetable operating authority was discontinued and train numbers did not mean anything for railroad operation.