Stall is like Einstein’s Law of Relativity. Absolute stall is variable with or relative to how much power is applied. Assume the out put is locked so no rotation can occur on that side, the stall will vary with the torque applied up to the point where the converter turned into a hydraulic granade.
So take your 1300 stall converter. Against an inline Chevy 6 developing 100 ft pounds of torque might be 1300 rpm and that torque number rpm where the converter stops any more engine RPM gain might be where the published stall comes from. Pinned between a post an a 305 V8 the same converter might show 1500 against a load equal to 130 hp while attached to a 350 developing 180 hp the stall for the same converter will be 1700 RPM. This is also related to the oil viscosity and temperature of the test. Plus there is always production variability while parts are interchangeable every part has a production tolerance how these stack up in any given converter is going to affect stall.
Torquing up against the vehicle’s brakes basically shows the engine RPM where the force transmitted by the axle exceeds the holding power of the brakes. It might or not reflect stall.