Tanks pushing/decreasing WN8

hey i have a question... when i drive T10 czech medium TVP 50 if decreases my wn8 EXTREMELY even if i drive over first gunmark permanently.

then i drive with t8 swedish medium for example or even with another t10 like 705A and wn8 goes up again.

why is tvp pushing down wn8 so hard? i really love this tank but i dont touch it since it ruins all my work