Make Will of the Ancients show up when you check your Spellvamp

Currently, when you upgrade from to your spellvamp drops from 12% to 0%. Why doesn't show up in spellvamp?

I mean, Last whisper shows up in armor pen even though it's BONUS armor pen. So say you're Darius, with a maxed E (25% total armor pen), and you buy a last whisper (30% bonus armor pen), your armor pen reads 55% (it adds the two even though they're diff kinds of armor pen). Why can't Will of the Ancients do the same?