It really sounds like you've gotten hold of some 12 exposure film rolls. Loaded carefully without using any more leader than absolutely necessary, one can get an extra shot out of them. It's the same thing with 24 and 36 exposure rolls. One can usually get 25 and 37 frames, respectively, with most 135 format cameras. Self-winding cameras that first appeared in the 1980s and use rubber pinch rollers to advance the film (rather than a takeup spool with a slit in the stem), can get 26-27 and 38-39 frames, respectively.
Until exposed, roll film is just a continuous long strip of unexposed film. The "frames" are created by the size of the camera's exposure area. The "gap" between frames is determined by the difference between the length of the camera's exposure mask/film gate and the length of the camera's film advance.¹ The frame numbers printed on the edge of most 135 format film sold in preloaded cassettes are just there for reference. There are no such marks on bulk film that many photographers bought to load their own cassettes.
For more about frame spacing on roll film, please see: Is the space between frames on film standardised?
It may even be possible that the film you are using was loaded from bulk film by a "Do It Yourselfer" who reused cassettes marked "24" or "36" exposure. Prior to 1934 when Kodak introduced the single use cassette everyone had to load their own roll film onto spools or cassettes that came with the camera. But there were also plenty of 8, 10, and 12 exposure rolls of 135 format film sold prepackaged back in the mid to late 20th century. Most such rolls were sold via commercial supply channels, rather than at retailers.
For who would desire such short rolls,and why, please see: What is the rationale for having less than 36 frames in a single 35mm film roll?
¹ The "standard" gap for 135 film is 2mm. 36mm + 2mm equals the 38mm distance used by exactly 8 sprocket holes in the 135 film format (35mm film). But some cameras use a tad more or less between each frame. Some even vary a little from one frame to the next!