I'm not sure "rarity" is necessarily the reason for an ENY of 20 for both planes.
I would suspect (perhaps the OP can verify) that the question is: "If ENY is an indication of a plane's performance, why do these two planes have the same ENY when their performance is quite different."
I could be all wet, though . . .
But assuming I am not, one must understand that ENY is basically set in increments of 5 (a few oddities like the 8 ENY 51D notwithstanding) leaving you only 8 levels (5 through 40) at which to put all the planes in the planeset. So, you will have planes with the same ENY that will be very different in performance.
Based on your flying style, the 38L might be a "20+" and the P-39Q a "20-", but their positions as 20 ENY planes are relative to the rest of the plane set. Neither are quite "bad" enough to be ENY 25 or "good" enough to be ENY 15 by whatever measuring stick HTC uses.