The firmware that is downloaded by the various Garmin webupdaters etc *is* the full firmware.
Your device will not have 'reverted' to a previous firmware; that's not retained.
What will most likely have happened that the firmware has become corrupt so that it's not passing muster as 'current and correct' by the firmware update checker, so it's offering you the current update again.
You can, if you wish, retain an off-line copy of the firmware package to re-install in the future without using any Garmin software or website.
The firmware update software does not in fact update the firmware: it just downloads it and dumps it on the device as X:\garmin\gupdate.gcd
It's on re-booting that the device discovers this file, validates it, un-packs it and installs it.
After it installs, it deletes the .gcd file.
You can browse to the garmin drive and grab a copy of the file after the updater has done the download, but before you unmount the device to re-boot.
You can re-apply the update simply by dumping the file back in the garmin folder of the device at any time.
( And you can revert to previous firmwares too, if you saved their .gcd files. )
The firmware updates do not touch the user files, as you have discovered. The 'full reset' does.