fury duration

[Vardelm.8539]

I'm playing a thief DD build with the sundering shade trait, which shows a fury duration of 7 3/4 seconds.  I also have runes of rage which give +30% fury duration.  That should give a fury duration of 10 seconds & change.  However, when testing on a training golem & mousing over my boon icons, the most time I see on the fury icon is 4 seconds.   Even assuming it takes a bit of time to appear so that it's actually starting at 5 seconds, that's still half of what I should get according to tooltips for traits & runes.