Topband: 2 channel SDR, Linrad, comparison of SNR of two antennas

[P H]

to those who watched this video: 
https://youtu.be/UzP03mZVLvE

can you help me in my search to resolve the puzzle: why is reception on 160 meters so much different compared to the other bands???
Flag antenna pattern is very much similar 1.8 up to 14 MHz (and even higher)
is the vertical polarization of the Flag sufficient explanation for the mediocre performance above 1.8 MHz???
Regards
Piotr, SP2BPD