It's possible, too, that something else is going on: You're working harder on CamelBak trips for some reason, so you sweat more and have less left to dump into your bladder. Otherwise, I'm skeptical of the notion that you make "better use" of sipped water. It's true that your kidneys will respond to large amounts of water by sending more to the bladder. But short of pounding down a quart or two at a time (most stomachs have a four-cup capacity, more or less), your body is very good at making use of any water it gets, depending on its need.
Anyway, it's an interesting scientific question. Maybe you can figure out a way to test this by monitoring your fluid intake for 24 hours as a control, then on subsequent Saturdays take the same hike at the same temperature and humidity, using a CamelBak one day, a water bottle another. Drink the same amount of water from each. Then pee into a cup and measure. There may be a Nobel in it for you!