I think what they're saying is to drink at least a gallon of water per day, not necessarily carry a gallon with you. As has been noted, it all depends upon water sources along the way, climate and, for me, whether I'll be hiking solo or with friends.

If I'm hiking with friends and there is water on route I might carry a liter and a half to start. Hiking the same route by myself and I'll probably start with as much a three liters. When hiking solo in the desert I usually carry at least two and a half gallons. Too many variables to answer this question in a simple manner.