Indicated MPG vs Actual

[LongBaz383BHP]

Not long had my XC90 D5 and was happy to see that on average it was doing 41.6 mpg. However I found that hard to believe so did a brim to brim consumption. 465 miles = 34.6 mpg. Why would the read out be so far out. I have to say I am happy with the actual mpg as have read some far more lower figures being quoted.
Barrie