I just went to download the offline installer for Cannon Fodder, then realized I had actually already downloaded it a few years ago. But then, I noticed that the currently available offline installer is for version 1.0, while the one from a few years ago is version 2.0.0.3

I checked a few other offline installers that I have downloaded, and it seems to be the same deal for them as well. Why is that?

Previously with GOG, the installer version was not the same as the game version.
It might be that installer version 1.0 is still game version 2.0.0.3

In most of these cases the offline installer(s) apparently just got repackaged to support cloud saves (as per the last entry in the corresponding changelogs).
This update could have been deemed important enough to also reset the versioning and start over at 1.0(?).

In case it's just the cloud saves update (again, by double-checking and confirming with the changelogs in the library) and you're neither using Galaxy nor its cloud saves functionality you can keep the old installer(s) instead of the newer 1.0 ones.
Might even save you a couple kilobytes of disk space.

GOG's old naming convention for offline installers used to be something like nameofgame_2.0.0.x. Their new one is like name_of_game_version.number_(build.number). Part of this is due to the "Galaxification" of offline installers (files used to be stored as files, but are instead now stored as "Galaxy streams" inside the installer), part of it is as Swedrami said, adding cloud saves. There have probably been no changes at all to the game files themselves (especially for DOS games for which most devs don't even have the 16-bit tool-kits / compilers to recompile for). If you don't want cloud saves (which don't even work for offline installers played without Galaxy), then you can happily keep the "old" 2.0.0.3 one. It's exactly the same game version though.

To add to this: the "2.0" and such in older installers (and I think even some current installers for older games that have never had to be updated) was GOG's installer build for that game, not the version of the game itself. Even with a lot of the newer installers for older games (say, early '00s and before), GOG don't always necessarily have info about/pay attention to which exact version they used. And in some cases where they did try to tag an older game with its actual version number, they messed up -- for example, everything I've read about GOG's build of (unmodded) Daggerfall from people who were already familiar with the game says that the GOG version is the latest official version (v. 1.07.213, commonly referred to as "v. 2.13" or "213"); GOG has labeled it -- both in the library and in the installer filename -- 1.07.

A few years ago now, GOG started updating games to a different installer, so that's probably all it is. A game like Cannon Fodder for instance is unlikely to get updates at this point in its life, so the versioning would be about the installer I reckon.

If you look at entries in many changelogs in your library, often all that updated at some point, was just the installer.