A simple test with guardian GS#2 (Whirling Wrath) with combat log open:
http://www.youtube.com/watch?v=0ru-aKLzHy0
Number above target is showing 290 yet if you add up the combat log it’s actually 364. The same is happening on many other fast-hitting skills, especially warrior GS#2 and GS#3. The damage numbers shown on screen most of the time are wrong.