No brass does not need to be "Worked in". You stated the obvious difference. The brass itself. Lapua brass is FAR better brass then any other on the market. They have more strict tolerances than the others out there. Thats why you pay what you do for that brass. As far as one outgrouping the other by such a minute margin, could not be contributed to the brass, just the shooter on a good or bad day.
Now, I would say that after 20-30 groups showing a group increase of over 1/2MOA or more, that would be enough to say that one was more apt than the other. It just may also be that that particular rifle just happend to like the way that load reacted with that brass due to its internal capacity for that load. Try a water volume test between the cases and see what you get.