The simplest explanation is that your motherboard's fan speed curve is not calibrated for your cooler setup, so it runs at less than 100% when it sees "high" temps or it throttles back the speed too early once it gets below 80, which keeps it at 80 instead of just max cooling. Many manufacturers don't set up the fan curves for max cooling but instead try to use a curve that gives good cooling within certain ranges without making it sound like your PC is about to take off from the deck of an aircraft carrier.

My intel reference board has settings for points along the curve specifically to allow for setups like this -- I'd be surprised if yours didn't have something similar.