It’s not, your reading the schematics wrong. The belt starts at 11” above the waterline and tapers or 7” below, if it tapered sideways in the diagram you posted it would have sections that are 11,10,9,8,7 but it goes straight from 11” to 8” then to 4” The central main belt is only listing maximum thickness. In the picture I posted it’s badly illustrated but the belt tapers downwards not amidship you end. Tapering belts isn’t a strictly AoN feature, though I will admit is happens most commonly on them.
Edit: here’s a much better image to detail the armor layout of HMS Dreadnought: