Cp will tell you the truth
After our airplane exercise it turned out that both our groups had the same 6 sigma factors butt our performances, savings and approaches were totally different! This meant that 6S factor is very vauge and can be misleading.
It's much better to have a look at Cp and Cpk. What's more we had an interesting situation that our process from Monday (which was totally underperforming) had a better Cp than improved process (which saved around 40k!).
Greame suggested that it might be because on Monday we had simpler tasks and had less time for i.e. mistakes, losing screws and the variation was smaller. And today we had lots of tasks and at least once sth went wrong shifting up the variation.
What I suggested as well is that on Monday the variation was smaller becasue we were operating all the time pretty much similarily - unfortunately way above the limit!! So despite learing the tasks we were hitting say ~4 minutes all the time, regardless our actions = variation showed it's consistent action. However, today we were rushing to meet the goal (which was not wrong at all, we weren't complaining about it and forced to do that maybe because we believed it's achievable) and we were better and better (on target to be honest) but it didn't work this way all the time - so I guess variation showed that to a greater extent... I can't prove it but that's my sense of this case and at least hope you can understand my way of thinking.