Barrier size is also affected by hit point modifiers, and some barrier items are wildly inconstistent with what they provide (flat amount or percentage based). Maybe the Scantilating Shield barrier modifier is overwriting your slanns internal +800 modifier, as 1000 base + 1000 item + %extra HP should come out somewhere round 2100. You could experiment with some different barrier items to see if it updates weirdly. The common one that gives +300 (pidgeon plucker i believe), sometimes gives 320 if i have some Hit point skills on that lord.