I had a similar problem I found it to be the friction washers on one side did not hold the shoes out and were pulled back by the springs. This caused one side to brake more while the other had a weak brake. I think the difference in your shoes are a new set and an old set which you can replace the friction washers, ones with the circlips. I checked the friction washers by prying them out to see if the shoes stayed out, in my case one set creeped back which meant more fluid was required to get the shoes out to brake. I had to buy a new set of shoes. Hope this helps
Ralph