Conjure Flame Bug?

I set my ranger up with a fiery bow string and set my fire attribute to 3. According to the skill description it says it'll add 5 extra dmg but when I use it, I see the bonus dmg of only 4. The same seems to be true for other attribute level. Seems to be always skill description minus 1 dmg.

Is this a bug or is that the way it suppose to be?

Fire damage does not ignore armor.

Thanks.