Darthsoilsguy,
With that concrete QC discrepancy. I've never had a conflict like that with another agency and never want to, but I live in Seattle, and a big factor in our compressive strength is temperature of the cure. It gets so cold here in the wintertime that even our cure boxes don't insulate things properly, and this affects our breaks, especially on the high-early or PT.
It seems like to me there must have been other factors. Probably a dead issue by now, but that type of thing still sets me off. Find the problem and fix it, don't have blaming the other guy your first mode of action.
It could have been awhile before their load cell was calibrated, or your load cell was calibrated, pads were changed, ect. They could have forgone a cure box, or left it out in the sun (I'm pretty sure some of ours do). Results for the same concrete will differ even from cylinder to cylinder of the same mud, but to me methods of making them are pretty much foolproof, especially when there's someone watching over your shoulder. Water under the bridge, but that water's comin' from somewhere...