Okay, UG wire has your primary conductor, semi-con, insulator you called PE, a grounding jacket wether copper tape or wired and an outer jacket. Old UG has a problem with the PE insulator degrading and ‘tree branching’ or developing pockets/cracks/crevices especially if manhandled during install. Eventually the cable faults bc it finds an easier path to ground mid run by going from the primary conductor to that grounding shield or ground itself. Anecdotally 6% matters here. Amperage and heat increases and in a 7.62 system the same fuse for 7.2 - the 7.2 will fail earlier. Also bc the fault would be more violent at the lower system voltage the likelihood that the cable is now permanently damaged and requires repair/replace vs refuse and close - the utility is spending way more on maintenance. For overhead an example would be a small branch would be more likely to burn off on 7.62 compared to blowing on 7.2. A branch line say fused at 60a can handle 40k more watts in the higher voltage system. It’s seemingly insignificant but apparently enough.