Windows 8 wasn't out when mIRC 7.27 was released, so mIRC simply doesn't know about it and will display the latest version it does know about, which is 7. I'm very sure the next stable release will fix this.

That said, I've always thought this could be handled better. Replying "7" is simply wrong in this case. IMO mIRC should display "Unknown" or something alike when it doesn't know the OS, rather than returning wrong info. It's not a big deal since these things are updated in later versions, but new mIRC releases don't always come as regularly as they have been lately, so I think this might be worth considering, especially since it's very likely an easy change.