That makes little sense... If you see an improvement on the ROS for 20Mhz then you should also see the same kind of improvement on 40Mhz. The only thing that happens in 40Mhz compared to 20Mhz is de widht of the data pata. Now (theoratically) you can send twice as much data. It's like having now a 4 lane motorway instead of 2.@20 MHz 802.11N (not AC) NV2 now 110 Mbit/s for TCP speedtest good job mikrotik . Now time for fix 40 MHz channel in this moment max speed only 150 Mbit for TCP
When you see this little improvement on the 40Mhz compared to 20Mhz as in your example then probably you have interference in the wide frequency. Where 20Mhz would have a pretty 'clear' channel the 40Mhz could well overlap with some other transmitting device which could ultimately make the connections even worse.
Because the radio energy now is devided over a twice a wide spectrum it means the power per hz goes down. And thus the S/N becomes smaller so the datarate cannot maintained at the same level and thus the capacity of the channel goes down.
So check if the 40Mhz channels is as free as the 20Mhz is....