Drag and Drop counter intuitive

Hi, I was wondering what the reason is for the following behaviour of drag and drop onto a folder:

If you drag a file onto a folder (in a tree) which is open and has subitems, the file will be dropped onto the parent of the folder. Only if the folder the file is dragged on to contains no subtimes or is closed, the file will actually dropped into that folder.

Why is the file not always dropped onto the folder, where ist was hovering over?