I think it has to do with the Layout mode your using for the groups. Grid as per preview is giving you something like a 400x400 model for those groups so I'm guessing there is some kind of truncation when its trying to map the pixel into one of those cells.
If you switch to "Minimal Grid" for Layout mode it will only define the size of the model group as the number of pixels it needs to encompass those models and that should make it work.