Their refurbs aren’t really “refurbished” in the typical sense, like it wasn’t used by a customer and then traded in or anything, those go to third party vendors rather than Apples own refurb site.
Very likely the studio display refurbs are boxes that were slightly damaged in transit but the display is fine, or a customer returned it after a few days, or the seal on the box was accidentally ripped in store so they sent it back, etc. They all have the same warranty as a brand new one and you can buy AppleCare, so there’s no downside to buying one really.