I know that this is a minimum guarantee but why was 2009/10 used when 2008/9 was used for the previous payment?
I'd guess because we were only half way through the 09/10 financial year when using the 08/09 figures was proposed.
Using the most recent figures available is the logical thing to do. Especially if its in your favour
Its also something any decent Union would have realised and used to their advantage.