It's an outdated discussion (but still interesting, maybe, who know, we will need to know in the future), but I am pretty sure armor was applied before too. When NPCs were hitting for, let say, an average of 400 unmitigated damage, total blocks events were also around 400.
It is still interesting for any potential Armor-Piercing effects that may be intetroduced in the future.
But I think you are probably right on the armor being first part, or we'ld see different blocked values.
Some testing done here is showing that popping SB is increasing your block chance AND your crit block chance by 25% each. This is completely separate from the "spillover" mechanic. No one was sure if it was a bug, or intended, haven't seen anything official about it yet.
Or a quick quote:
A bug has been fixed where the Shield Block ability granted 25% more critical block chance than was intended for Protection warriors with very high total chance to avoid attacks."
So yea, was a bug, getting fixed. Good bye 25% free crit block.