Your also in a fixed location from having a desktop while laptops are now geared to see the go anywhere connect anywhere technology in use while on the go. If the laptop is being moved around from room to room that would also explain why the "sweet spots" are being picked up on.
Your desktop on the other hand sees the antennae on the back side of the case in one spot all the time where it may not be the idea location in relartionship to the wireless router in use. In other words you need to almost park the router on top of the case to have it in the close proximately needed for an improved connection.
The card in the laptop likely is a bit more sensitive since it was intended for portable use which can also explain the better signal strength seen while the card on your desktop is the average PCI type most likely.
NetGear is usually one of the best to use over something like the Cisco Linksys wired/wireless combo router here where a second system was still not able to be added on unless hardwired!
That would have allowed the use of the wireless pci and usb type adapters for working on other systems without always a need to resync everything when swapping over from one and then back to the main case.