I've just noticed the same thing happening with "Bracers of All-consuming rage" and "Snerf's Folly" on The Watcher. Some stats rise, others don't.
Is this some kind of known issue with the game, or is there some kind of stat-raising limitations in the game mechanics that aren't being explained to us?
I'm having a similar issue, except in my case it is with the Watcher. The amulet says it provides +2 int and +1 res, but while wearing it only +2 int takes effect, and res does not rise at all.
Screenshot uploaded as evidence.