The first three are only links (or pointers) the last one is an actual file.
When you remove the first three from the gDrive folder Google will actually move those documents to the trash and your link becomes kinda useless, when you move the last one from the gDrive folder Google still moves the file to your trash (in gDrive) but because it's a real file, you still have it. Google does this because they don't offer a true offline viewer/editor and thus it makes no sense (to them) to take up offline space.
With dropbox all files are considered real and therefore when you remove them from dropbox you retain the file even if it's no longer synced.
While skydrive has office docs, they too are stored as real files on your computer and thus if you remove them from the skydrive folder you get to keep the file even if it's moved to the rB in skydrive.
note: (in the screenshot) those check-marks might be skydrive check-marks and not google's, this is because my gDrive folder is inside my skydrive folder for redundancy -- it still wont sync gDocs, but since I don't really use gDocs I'm okay.