Don't know if I'd say "on the fly," but certainly I'd agree that it is likely symptomatic of early multicamera shoots. They were still learning how to do it back then - both the camera operators / focus pullers and the talent - and it is very likely that the focus puller either...
Have made it all the way through the first two seasons and have been having a blast doing so. What I found interesting, especially in the first season, was how often a shot was just slightly out of focus. Probably noticed this on the Moviola in the edit bay, but certainly it was never noticeable...