I say that it is a likely enough explanation because I can't quite imagine what else is changing in the hubs' setup.

This given that I can say with some reasonable assurance that neither lock nuts or skewers changed -- on account of my having personally tightened the nuts and the skewers on all occasions. And though I cannot be quite as sure that spoke tension couldn't have influenced the bearing adjustment (I did start off by adjusting the hubs straight from the box before building the wheels), it is definitely part of my standard operating procedure to check for play or tightness or any other type of wonkyness when mounting new or overhauled wheels, so I'm assuming that had there been this kind of lateral play at the rim when I first mounted the wheel, I would have gone back to adjust it out.

As for throwing out what 'I thought I knew about cup and cone ball bearing durability' -- I say that on account of my impression that I could expect many thousands of miles of use from maintained hubs of this sort. And it would seem contradictory to this expectation if the bearings' play resulted from a few hundred miles' wear. How durable could these things turn out to be if the races and/or balls are already significantly worn after so little use?

That said, I built up a rear wheel around a similar XT hub last year and after putting a few thousand miles on it I did find some wear marks when I overhauled it earlier this summer -- slightly more evident on the cones than on the cups as I expected, (and I replaced the balls per routine at that point). And I'll admit to being extra prissy with these new hubs in an attempt to gauge just how often I need to open them up and replace the balls if I want to maximally preserve the cups and, to a lesser extent the cones.

One thing which is quite interesting to me at the moment is that the fronts seem to develop lateral play at the rim faster than the rears.

That's a thought. I did in fact build the wheels right around the hottest part of the year. However, it was not much cooler when I switched out the first wheel a several weeks ago. Luckily, the difference thermal expansion easy enough to quantify; so I will figure out exactly how much difference there would be in the contraction of the aluminum shell and the steel axle across a 10 or 20 degree temperature difference.

Thanks for the input

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In any case, in a effort to operate with as much good information as possible, I am going to go inspect the cups and cones of the second, still unadjusted front wheel, right this minute.