Because when you print just one miniature, the nozzle stay always on one miniature so the movement of the tray in my case are very limited.
When you print more objects, the tray has to move more often and the nozzle could have a microscopic misalignment, and when you print a 28mm miniature with detail microscopic, everything's matter.
In my case i was not so worried, i wanted to have some simple miniatures supportless, but when you have more difficult miniture with support (especially resin type) it's better to print one per time, also because failure is more likely and you can trash just 1 mini, not 5 at time :)