Let's say an external drive has been assigned the F: letter.
You unplug the drive and for some reason, you plug in another kind of device. That new device will be assigned the F: letter and if you plug in the external drive, it will receive the letter G:
That's a Windows feature.
In that situation, the drive letter stored in the catalog is not updated.
However, the catalog does not only store the drive letter to identify it, it also stores the internal drive number assigned by Windows, which explains the files are still accessible.
I would not recommend trusting that feature, it may create ulterior problems for backups for instance. So do like advanced users of Windows: assign a letter farther from F: in the alphabet, like S, T, X... After that, that manual drive letter will always be recognized by Windows, independently from the other plugged in devices.