2003 Setup and Booted OS show different system Drive Letter

Just re-installed Windows 2003 Server on one of my development servers. Has 2 hard drives, one is D (made no changes to this disk). The system drive (c:) I formatted and reinstalled Windows Server. During setup, it showed as C:, and everything seemed normal.

Post-install the booted system shows F: as the system drive (D: is still there and intact). I re-ran setup — booted from CD — and setup still shows C: as the primary/system drive.

Why does Windows show it as F: after booting, but C: during setup?

(there is NO c: in the system when booted)