I wonder if theres a bit more to this than meets the eye.
Undoubtedly.
I used to be quite an expert on Microsoft licensing, though mainly in the corporate world, and they quite likely charge "manufacturers" different fees for OEM copies depending on how they're shipped/installed.
e.g. a preinstalled copy on a hard disk (with no recovery media) could well be cheaper than one also shipped with the media.
I have noticed that many recent systems not only come preinstalled but also have no means to create recovery media; the only option you have is the extremely tedious one of taking a full system backup.
No doubt those copies are even cheaper.