Hello,
Sorry to answer late,
1st of all i tough :
In the F15 you alitmer setting is 29.92 (=1013 Hpa).
On your HUD you are AT FL120.
If you were using a QNH you would read 12500 feet ( or 11500 ft, depend on the weather condition of the day ).
But after testing it, i wounder if it is just that like in reality the altimeter is less accurate with altitude. But here it is a real bad altimer !!
I think the altitude in F2 view is the QNH altitude. But in the f15 i don't know which setting we are using.