Now, it is entirely possible that this is the way the game is supposed to work, but I'll put it here anyway.
I've got this nice bit of loot with Spell Immunity: Web on it. I just fought a bunch of Hunting Spiders, and it grants no immunity whatsoever to their web bolt attack.
This may be a case similar to petrification gaze and drown, but I just thought I'd list it.
It's spell immunity to web the spell.
Correct. Web Bolt != Web Spell, and there is no web effect AFAIK. Freedom, however, should give you immunity to the effects that happen in the Web spell (and bolt).