Falling liquid gains volume as time goes by. (The amount of water that lands will be greater than that that fell.) This can be observed if a player starts a Hellevator with trace amount of water; the level will slowly rise until player needs to jump to breathe. This also happens with lava but is harder to record due to its slow movement and the need for potions for the player's protection. This is likely caused by the game calculating from the graphics and not literal amounts.
Placing water above a single block will separate the fluids in two amounts that can be recollected by a bucket. A good visual is of a lower-case n or upside-down U or J.
A 3 x 1 (1 tile deep, 3 tiles wide) hole filled with a single bucket can collect from one side and leave a (buggy, frozen in place until a nearby block change) half left over.
Similarly, a 9 x 1 hole with a 2-tile deep center will provide more water in total than was used to fill it up. After using a bucket and taking from the lowest point, there will be small bit of water left over.
Fluid drained from a source outside rendering or monster spawn range will never run out. Distance can be tested by trapping a monster in a hole and walking away a certain distance until the foe disappears. It varies by window size.