Yes the gain is slightly lower than 39dB, it works best with high output MM, greater than 5mV...
Also AT is not a good match on my side...
No idea about the reason (capacitance maybe), anyway I don't care about the measures for this pricepoint.
About the slight roll off it is there and can be listened, it is a warm sounding unit out of the box, maybe a bit "dry".
I've found improvements in both top end and mid-bass swapping OP Amps, but this is considered a shame... So ignore this comment if you're an OP amp hater...
For MM I've found a nice spot using a Jico J44A-7 which has a ~8mV output.
So my suggestion is to look at the DJ/Clubbing carts.
Otherwise use the switches on the base even with MM, and move gain at 9/10 o'clock.