I know.. and dude on swap and go truck while back said they are going to go to 1 size in 9kg and 1 size in 4kg bottles soon.. so will make it harder for all unless changing cylinder brackets are cheap
For them to do that would require 2 things to achieve this outcome
1, rationalise all their existing stock including the 3 or 4 in my yard and the other thousands of bottles out in service, achievable yes but would take some time.
2, refuse to except the current practice of owner bottles in all shapes.
Just thought of a 3rd reason, the Bunnings staff along with the Indian servo attendant would need to check the hand in bottles against some form of bottle gauge. I can't see that happening, it is hard enough find staff to help you at Bunnings and the servo attendant normally just gives you the key to the cage for self service as he or she can't leave the service console.