Ah; well, that explains it then. The site truly is an anti-XP (and Vista) bigot; you just chose to "pass" as Win 10. Probably not even realizing you needed to.
Pray tell, how do you fake your display resolution, given that it's not part of the UA? Asking for a friend
Seriously; faking a display resolution would help with browser fingerprinting, for those of us using Microsoft's VM. In full-screen mode, the display resolution is usually your monitor's resolution minus two rows. That "minus two rows" stands out like a sore thumb.
I don't personally, but a lot of folks do.
BTW, this is a great example of "hyper-resolution-itis;" a disease which inflicts many designers of smart phone and laptop screens these days: The compulsion to cram as many pixels as possible into as small a rectangle as possible.
Come on, let's get real: no one can see the difference between 1280x720 and anything greater, on a screen the size of a typical smart phone, unless they're viewing the screen with a microscope! Even my old BlackBerry Priv (well, ca. 2015 is "old" when you're talking about smart phones) has a 2560x1440 screen! (I like the phone anyway.)
But "4K" resolution lets the marketing guys brag: "You need our phone because we have more pixels than the other guys!!" And enough folks are stupid enough to fall for it, that the disease has become endemic.