Odd lighting discovery, Any thoughts?

[Delphinus]

Maybe the fact that the bulb got shook up, moved around a little when taking it out and back, somehow affected the arc tube? Maybe we need to make it a point of routine to shake our lamps a little every couple of months? Wait, is it better now that it's different, or worse?

Or ... hmmm, wait, yeah you said you're using electronic ballasts. I wonder if a lamp just gets "used" to a particular ballast, and after switching them, they have to sort of re-burn in? Maybe it's just enough of a difference that even switching back to the original ballast is still different enough now that it still needs to go through the re-burn-in phase?

(Just grasping at straws here really ... it's rather quite a bizarre observation!)

[Static]

just a suggestion, but maybe you've turned the bulb upside down, so the part facing the top is now on the bottom and that made a difference somehow?