Ranger - Regeneration
30 points in Nature Magic gives +30% boon duration. 3 seconds base duration * 1.3 = 3.9 seconds of regeneration. This is closer to 4 then to 3 3/4, so the tooltip would probably round to 4 seconds. If regeneration provides healing every second (like how bleed works), it would still only provide three ticks of healing, and therefore heal the same amount as without the boon duration. However, it would not be any weaker, because it is still doing all that healing within the first 3 seconds – you just have an extra .9 seconds of having the buff but not receiving any healing in that time.
It’s still not answering the second thing.