My personnal/fluff understanding about that :
The guys on the battlefield saw so many horror (mutilate body, friend explosing on the mine...) you could understand they are now rough guys, no more boy scout.
They panic/fall in madness not easily now after seeing all the horror of the war. But when they are near the burnout (suppres token) the horrors of mythos do the job against their mental sanity.
Yes, it makes some sense.
What does not is that ways to deal supression are very limited.
So it is well possible to have a Mythos army that has absolutely
no chance to give you any scares (no machineguns etc.).
And that sounds bad.