I think it quite common for cables not to fit properly.
One issue is that a number of cables currently being sold have moulded solder nipples mening the cable cannot be shortened without having a brass nipple to solder in place.
Rear brake cables are a difficult to find in the right length. There were 4 different ones made by BSA. One for Ariel type hubs. One for the early 1958 iron hubs, a different one for the later 1958 iron hubs due to the repositioning of the square in the hub lever, and a fourth for 1960 on with the hub lever pointing down.
Correct fitted length of rear cable depends on the position of the left side footrest and the psoition of the brake pedal on the splines in relation to the short lever the other end of the crossover shaft.