You aren’t doing anything wrong, you have the wrong expectations 
75% throttle isn’t the same as 75% power at higher altitudes.
At 9000ft you need almost 100% throttle to achive 75% power.
According to the manual 2320RPM at this altitude means slightly more than 50% power at ~104ktas.
So the MSFS version is actually very close.
106kias at 9000ft means 121ktas, the manual shows ~123ktas at full throttle.
So these numbers are matching nicely as well.