"The amount of water estimated to have been drunk by all humans that have existed is a tiny fraction of all the world's water"
Indeed, but surely some bodies of water get recycled more often than others? For example, some bottled mineral water (such as that in Buxton) is estimated to have been sat underground for 10 000 years or more before bottling. Whereas I imagine shallow lakes, man-made reservoirs and the like will end up in the loop more often as rain will fall in the mountains and refill said shallow lakes etc.
I could be wrong about this, of course. Much sleep has been had and much beer has been consumed and subsequently returned to the planet since my Geography lessons of 20 years ago...