Modern coaching stock has tapered ends so that the corners won't swing out so much when going around curves.
Most EMUs have 20m-long coaches that only need a small angle on the corner, while SWT's 444s and the Mk3/4s all have 23m long coaches and so have more of a noticeable wedge cut off.
The IEPs have 26m bodyshells - a first for the UK - with the same spacing between bogies as a 23m Mk3 and hence very long end overhangs. That gives them a very noticeably tapered end that can't be used easily for seating, hence the rather odd interior layout.
The curvedness is just the effect of chopping a vertical slice at an angle across the curved bodyside profile of the stock.
Thanks for the explanation. That'll stop bugging me now