Part of the trouble with flight sims is they can't model every variation of aircraft.
You say the have the P-39N,
Now which Yak-9?
Yak-9
Yak-9T
Yak-9D
Yak-9 with M-106 engine
Yak-9P
Yak-9TK
and that is just the Yaks that showed up in 1943.
Six more Yak-9s showed up in 1944 and some in 1945.
Performance in sims is often approximate and sometimes "tweaked" to make the "Game" more playable, or to appeal to a larger audience.
Not that they are going to tell the customers that on the box
As has been noted, the guns are often way simplified to make game design easier.