A few things:
1-Different panels with different judges. As you were saying, individual tastes vary.
2-Different performances. It is not as though your favorite group has a wide variety of performances while everyone else is constant.
3-Scores are relative and especially affected by nearby groups. I’m not sophisticated enough to determine performance intricacies on grade 6 material, but I know O’Fallon seemed to blow the paint off the stadium in prelims but seemed a touch dull in finals. I suspect that comes from comparison as much as performance.
Ultimately, it is an art, not a science, and the psychological variables are numerous and complex.