I think I see the confusion here, and I'll make sure it's clear(er) in the BRP book.
For normal use, shields shouldn't have their armor values applied twice, which is essentially what you're doing here (once as a armor value, once as HP).
Generally, it's only when you're dealing with specials and criticals that the shield's AP or HP are even an issue. These are ablative, as well, so a damaged shield has a lower AP for subsequent attacks.
The HP for shields came from Stormbringer, or were extrapolated using those values as a benchmark. And for clarification, RQ was the third tier of reference for this book. In order, the first source was Elric!/Stormbringer, then Call of Cthulhu, then RQ was utilized where those works didn't suffice. Then came Elfquest, Ringworld, Superworld, etc. So things aren't supposed to work exactly the way they do in RQ.
I must confess to a bit of frustration that, due to factors beyond my control (the release of the old RQ3 stuff as the BRP monographs, for example) it's been assumed that this work is somehow meant to fill the same role of RQ. If you'll note, many of the optional systems are those from RQ, while the default is a gameplay style more inspired by Stormbringer and Call of Cthulhu.