I think the difference is in the testing
http://www.sterlingrope.com/media/docum ... manual.pdfImpact Force: Tested during the first factor 1.739 fall (4meter fall on 2.3meters of rope).
- Single Ropes: Impact must be less 12 kilonewtons (kN) using an 80kilogram mass.
- Twin Ropes: Same as Single ropes, but using both strands during the test.
- Half Ropes: Impact must be less than 8kN, tested using a 55 kilogram mass.
Falls Held: Number of falls rope holds before breaking.
- Single Ropes: Must hold 5 falls of an 80kg mass.
- Twin: Must hold 12 falls of an 80kg mass. {I'm quite sure this is also with both ropes tested together}
- Half: Must hold 5 falls of a 55kg mass.
I rarely see half ropes that are less than 8.5 mm diameter; but often see twins 8mm or less.
I'm sure that rope makers test their ropes before submitting them to UIAA; they would probably love to have 7.8 mm ropes that pass the half tests, but if they have doubts they will pass, the rope is submitted as a twin.
On dynamic ropes, the impact force rating is
NOT a breaking strength rating; it's an approximation of the maximum force a climber will feel at the end of the standard (test) fall. This concept originated long ago, with the testing of parachute harnesses. If a rope stops a fall and doesn't break, but breaks your spine or pelvis, that's still not a good thing.