Hi Fionnán
As Michael says a lot of different factors. You have to also remember that barrels will not necessarily hold exactly the same amount from a manufacturing point of view i.e. the 250l is a close estimate. Also or an evaporation point of view or an age of maturation point of view barrel loose different amounts even if of the same age.
Have a look at these
Cadenhead Bottles of Cooley from my site to give you an idea.
The run no column is the amount of bottles. These are all bottled at the natural cask strength of each cask.