so is that about right or do I have some thing wrong?
It might be to do with your power commander.
Did you have it set up on a dyno or did you just use the map that was installed?
The power commander can be programmed for better performance or economy so a different map may make all the difference.
Naturally if you get it set up on a dyno it will cost £££'s but may repay itself in better economy in time. You could always disconnect the P/C and see what difference it makes. There are usually maps available to download to suit the end can etc, might take a bit of hunting to find one.
It could be down to your riding style and how much town riding you do, it could also indicate a blocked air filter, knackered spark plugs etc - I had a bike serviced even though I thought it was fine and my mpg went from 32 to 46 and I still rode like an idiot!