Wrong analogy. Imagine two buckets, each with a hose connected to a pump. If one bucket has 1 gallon of water and the other has .75 gallons the .75 gallon bucket is going to run dry sooner.

Also, when cells are connected in series there seems to be some slight differences in discharge rate depending on where in the pack a cell is. Not sure why but with NiMH and Nicd it always seems like the cell with the negative power tap dumps first.