Logically I'd have to agree, but both the bearings were solidly driven in. I conclude that the axle is not the original, and the shoulder-to-shoulder distance is wrong for this hub, and is really supposed to be 71.

I removed a bearing and added a 1mm spacer between the axle shoulder and the bearing race, on the non-drive side; drove the bearing in again, tightened the locknuts and it feels perfect. I'm going with this slightly dodgy fix. And next time I won't buy used wheels with unknown history.