Question about level 20 leadership - destroy enemy camp

I recently got leadership to level 20 and started doing the destroy enemy camp one. I am a bit confused on how the speed boosts work with the profession assets. I thought the adventurer would reduce the completion time by 25%, so this 12 hour task with one adventurer slotted would bring it down to 9 hours. But it only brings it down to 9 hrs 36 minutes. I then slot in a second adventurer thinking it would then be a total of 50% reduction and bring it down to 6 hours, but it only brings it down to 8 hours. So are the adventurer's bugged or is the task bugged or am I just not figuring this out correctly.