How to read battleship belt-armor figures

Did Queen Elizabeth have a uniform 13-inch belt while Nevada's armor varied under an all-or-nothing scheme?

No. Belt armor on both ships varied by height and position, and quoted maximum thickness usually describes only the main waterline portion.

A battleship's belt was not one uniform slab extending over the entire side. The main waterline belt can have a quoted maximum while upper belts, lower belts, casemate protection, and the forward and after ends use different thicknesses. Even ships associated with all-or-nothing protection require attention to exactly which part of the belt a source is measuring. Summary tables can flatten a complex three-dimensional armor scheme into a misleading single number.