When I set the MAC OSX version filter to 10.6.8, and look at results from my library,
it is filled with results of 10.7 and above (alongside of the 10.6.8 titles). Why is this?
What then is the point of the MAC OSX version filter?

There's no point; it hasn't even been called "OS X" for many years now. Just GOG being GOG. Also, 32-bit Mac games don't show up under the "OS X" filters at all. All Mac games currently being sold as Mac compatible are 64-bit and should theoretically run on any current macOS version, so filtering by "OS X" would work. If you're using a really old version of the OS, though, the filters are useless.

Maybe GOG had planned to put out something for the PPC emulators out there, or the very odd 68k game.

That would make no sense (you'd need a filter for CPU, not OS), and any Macs that used 68K processors ran an entirely different OS that basically has nothing to do with the current macOS starting with 10.10 other than the name. You'd need an emulator to run them, and get Apple's permission for the required ROM images and OS. It's unfortunate that will never happen, as some games in the 90s had better Mac versions compared to DOS as they tended to run at higher resolutions (e.g. Warcraft 1, Prince of Persia 1+2, Dark Forces, etc.). Then again, if GOG was interested in the best versions of old games they'd replace most of the older DOS games with the Amiga versions, or even C64 versions in some cases.

Not sure if it's related, but GOG identifies macOS entries as "osx" under most of its API endpoints. I doubt this naming quirk will change, for backwards compatibility reasons.

Original Pax Imperia would've been worth it.

I have a good condition Intel mac-mini running Snow Leopard.
I'd like to load it up with compatible games. GoG offers Mac games, and ostensibly allows sorting them by compatible OS version——except it doesn't work; the sort filter seems to be ignored when assembling the results.

I cannot state how many times that I have filtered for ver. Mac 10.6, and then downloaded from the results, only to find out (after wasting the metered bandwidth!) that the titles won't install on my OSX 10.6.8 mac. What is the issue?

Is it that they are not compatible with version 10.6, or is it that they are, but only run on PPC macs, or is GoG's website just broken?

Yes, it's broken. GOG has never sold any PPC-compatible games.

...And nobody asked for them. I have an Intel mac; as stated in the above post. PPC is not the issue.
The issue is that they offer an OS version filter... presumably to filter the OS version (requirement).
But it doesn't do this—hence my original question.

Rephrased: I wanted a listing from my library of just those games that would work on Mac OSX 10.6; the results always include titles that wont.

I have an Intel mac—I mean I do have PPC based macs, but I didn't mention them before now.

Thank you for the confirmation that GoG's filter feature is broken. It's a shame that they haven't bothered
to fix it in all this time; it looks bad on them. Hopefully some day they will fix their site.