I have never seen this. I pulled up a mobile site on my desktop and it showed me like how it would look on mobile. See screenshot.
Any clue of why it would do this?
What pulled up.
Don't really understand what you are asking here.
It does seem a bit odd to me.
I wouldn't expect going to a non-mobile site using a mobile device to give me a "desktop preview".
Maybe I'm so used to thinking "one responsive site for all" that I'm missing something about mobile specific sites?
Looks like its a site created by something like this.