It's my opinion that it's mostly related to the quality of the site. I can't tell you how many problems you can Google for and your first ten options for answers are on the stack network.
In my experience:
Specific Debian derived error messages mostly point here.
Really intelligently configured Google queries tend to end up here or at the arch-wiki.
Just a personal note:
The second subset of people that know it's a fork/derivative, should, probably, also understand without asking that there are very significant differences between an upstream distro and a forked distro. They just seem to ask anyway... Specifically when the entire GUI system is lopped off and replaced (like the Mint -> Ubuntu with Unity -> Debian with Gnome 3/KDE/some lightweight window manager).
