I am no expert but I have read that that stringing was standard and when I thought about it it had a couple of benefits it seemed to me...
One was that the roughly equal full (or empty) weight bottles would ultimately reach an equilibrium and level themselves out. The shortened string (shorter than when you tug it) should make it more difficult for the cap to work its way up and off thereby exposing the charge to spilling, fire and sparks, rain or snow.
Also, as there are two separate strings per bottle, when a single string breaks (and they do -- waxed flax I believe) you do not lose a bottle. Both strings on a bottle would have to surprise you and break to lose at least one of them.
I have also see them with rings connecting two adjacent strings -- this too would probably help keep the string effectively "short" above a powder bottle top unless someone decisively pulled up that bottle to pop its top (which is so much fun).