I think that it is an anomaly. Same as the water has the biggest density at 4°C instead of 0°C, another density anomaly is when you put water in your backpack.
I switched to water on a cheap plastic bicycle bottle clip modified (with a hot knife blade) to clip onto my belt. Usually around back, out of the way and far less noticeable than being in a knapsack.
But lately, I’ve been contemplating a bike racer’s water-bladder, inspired by my bikeracing fanatic son-in-law. Just a tube to sip – no messing with bottles, etc.