Why don´t these figures match up?

[MusseMus]

I did a little test in FMB, where I let 2 AI planes fly next to each other for some miles. One was a 109E-4 and the other a Spit 1.
I measured the time it took for them to travel 20 km and I checked their speed gaugets (AI on).

Observation 1: The 109E-4 outran the Spit on every try No idea why they did not match speed since I used exactly the same settings on them.

Observation 2:
The airspeed gauge on the 109 red 310 kph, but the calculation gave me 325 kph=a difference ofabout 5%

The spits gauge showd 170 mph=273 kph, but the calculations gave me 316 kph=a difference of about 15%

I'm aware that I'm comparing IAS and ground speed here, but the difference at 500 meters should not be this big, right?

Comparing theese results with the table in post: http://forum.1cpublishing.eu/showthread.php?t=34115

Coincidence?

/m