Good explanation by Bealzebub.
One error we had was when the old boy doing the refuelling used the wrong SG page.*
The energy in our fuel load is proportional to its mass, not its volume and the mistake on this occasion was to load the volume for a higher SG than that of the actual fuel. The refuelling system measures volume loaded.
As with so many of these things the resolution, although apparently simple, turned out to be less so resulting in a bit of paperwork for Bas.
But fear not; we departed with the correct fuel on board.
*A habit had grown amongst the refuellers of placing the most common SG page at the front and not in sequence. Someone had looked at our on-board copy, probably a pilot, and thought 'That's wrong', opened the ring-binder and placed the page in its correct order. The refueller just used the figures from the first page without looking at the SG at the top to which they related.
The RAF had large signs in the hangars which read 'Don't assume - check!' There but for the grace etc . . .