It's nothing to do with how many days they pay you for travelling or a change to which route they calculate you have taken. It's just that the rate has gone down, simple as that; explanation, "The price of fuel has come down".
This means i'm about £150 a year worse off, compounded by the fact that fuel has actually gone up.
You probably wouldn't notice £150 over a year, but it smarts a bit when backed up with complete bo11ock5.