My guess is that what feels like a perfectly adjusted hubs in your hand is actually an ever so slightly lose one. once the wheel is installed the play that is in the hub is more obvious because holding the rim and shaking gives you much more leverage than simply trying to test the axle between your fingers.
Generally when adjusting a QR hub, installing will actually tighten it a bit. I think this is because it squeezes in that tiny bit a space of the threads that makes threading possible. But that theory may be really off the mark.
I would try micro adjusting the tightness of the cone, checking it at every 5 degrees or so. Find the spot where it is actually too tight to spin smoothly with your fingers and ten back off ever so slightly.
Also, it is very important to tighten the cone and locknut against each other. Otherwise the hub will become unadjusted very quickly.
If the hub has been damaged or worn from use, there may be no perfect adjustment. look for pitting, or dimples in the cone, the hub itself, and the bearings. if that is the case you can replace the easier to replace parts( ie, cones and bearings) replace the whole hub/wheel, or ride an imperfect wheel. It will probably be frowned upon here, but i would say that if you decided to ride it damaged, go for over tightening ever so slightly. It will further the damage and be a short term solution, but it beats a lose hub.