Board Computer Mpg versus Main Display Mpg

Since having a new 'Dashboard Speaker Grille' fitted, the average mpg on the 'main display' won't go above 21 - 22mpg yet the Board Computer shows in the 30's mpg at the same time - anyone noticed discrepancies between these two readings? (Or am I misunderstanding something - although the average mpg on the main display before the new unit was fitted was more variable and occasionally went towards the 30's mpg!?) Having gone through the traumatic job of trying to eliminate the noises & rattles after the replacement grille was fitted I couldn't face the prospect of it having to be removed again! (Antara Exclusive Dec.2012 AWD Auto)