This is not a bug, and there is no difference in the way Finder handles icons
for file sharing. With the "Always snap to grid" feature turned on in the
Views control panel, it is possible for icons to share the same place on
the grid. This feature allows more items to fit in a window. If you
prefer not to stack icons, you can turn off the "snap to grid" feature.
Here's how to "unstack" the icons:
1) Choose "by Icon" or "by Small Icon" from the View menu.
2) Hold down the Option key while choosing Clean Up by Name from the
Special menu.
We were able to duplicate icon stacking when we dropped the documents
one-by-one into an open folder at approximately the same location with snap
to grid turned on. The result was the same with either straight or
staggered grid. Icon stacking does not happen when moving documents to
closed folders.