Yeah its got me stumped too, I'm pretty sure the bearings are all the way down on the diff, spent all saturday looking at them, I can't get my thinnest feeler gauge between the back of the bearing and the diff on either side, and they look seated. (I stupidly chipped one of the races and had to swap out that and the bearing which is why the freeplay figure has changed).
I can't think of anything else I can do, considering that when I put new bearings in my spare box with standard diff the free play measurement came out fine and I used the same tools/methods to put it all together and measure it.
Yeah the reading is 0.68mm so I think i'm going to order the 1mm washer and hope for the best.