in which release are you ?
this indicator’s formula is based on a total amount of volume, divided by a time. Check those two values, see which one is “weird”. Maybe you’ll be able to investigate where is the fault coming from.
since B10, you can check the LLC throughput distribution (in the report mono object distribution), which is probably more precise.
i don’t recall any issue with the average throughput per tbf though… but i never checked it as you did 🙂