Oh, found this (funny, having similar conversation with a pixel artist for some custom work on a website - which we are using the 60/30 isometric view):
I remember a conversation some time ago about perspective and that the UO's art/map is more similar to the Military perspective as it uses a 45-degree angle on two side of the object being drawn - also called Plan Oblique - as in for 3D-ish display of building plans (for you architeches and autocad users out there). So technically, the full circle view is accurate for the perspective used. The things you learn in graphic design. ^.^
What I will add is that the foreshortening, shading and texturing of the art is off and what makes it seem all funky to us whenever new items are released. Without the foreshortening, it looks like we are seeing things straight on and our brains go, "huh?!" However this is limited by the amount of pixels they can use - so I can see the dilemma. There was a project many years ago where someone had redone the perspectives and a whole lot of art for UO that actually made sense. Am hard pressed to find that again - but I remember it very well.