Rotor/hub seperation?

[ExpySport]

I was rotating the tires on my '02 Expedition this morning and came across something fishy. Correct me if I am wrong, but isn't the front hub & rotor supposed to be one piece? On the passenger front side, once I took the tire/wheel off, I found the rotor was loose - wobbled on the studs. If the caliper were removed, it almost seems like the rotor would slide right off of the hub. When I pulled the driver's side front, the rotor was solid & didn't move at all. Is this the way it was designed or has the rotor seperated from the hub? Is this a common problem in the Expeditions? The tire/wheel is holding the assembly in place and I've noticed no drivability problems today. Is this something I need to have repaired/replaced right away or am I safe for a little? Common sense is telling me it's not right and to have it fixed ASAP. Thanks to all for your replies.

[tjc transport]

rotor and hub are seperate parts. the driver side is tight cause it is rusted on.

[ExpySport]

Thanks for the reply and info. I feel a lot better now!