Your use of the word 'bar' probably explains it. One meaning of the noun 'jump' is 'an obstacle to be cleared' as in high jump......"He's faced with a jump of 1.8 metres to qualify." Also, the device used to mark the height to be attained is marked by a 'bar'.
So, the 'jump' in a pub, for example, is the counter over which the staff might have to leap to stop a fight or - alternatively - the obstacle anyone intent on getting behind the bar has to clear to get there.
Thanks....was just wondering if there may not also be an answer based in the coaching inn days. I understand your answer and have to say alot of other people have said similar