Firkin fill level

[John Holgreaves]

A firkin is said to hold 41 litres of beer. However a firkin will actually hold 44 litres of beer.
Is the extra space there to allow a 3 litre air/CO2 space, or fill it and allow for 3 litres of trub, cloudy beer etc.?

 

[CJV8]

Possibly an allowance for ullage (the crud at the bottom), as I was always taught "fill to spill" for racking to cask.