I'm not in expert in the placement algorithm but the I think the placer will cluster cells from modules that have alot of connectivity or timing critical paths in order to reduce parasitics or congestion during routing. This is why it will create cell clusters and then leave parts of the floorplan unused. When utilization is over 100%, strange things happen (placer gets confused?) so don't make any assumptions from placement results when the design utilization is over 100%.
Try using the option: setPlaceMode -modulePadding <module name(s)> <padding factor>
This is what I typicaly use this to spread out cell placements.