LEM hy
I thinkyou figured it right: the speed will create ,probably ,a suction effect,and remember also that there are more than one valves open now.Also the negative pressure relieve valves.So ,probably,due to aerodinamics,and shape of fuselage,the air will be sucked out through
some of the valves (ex forward outflow,or flow control valve ) and because of the depression in the fuselage will enter through other openings (ex main outflow valve)Position of the valves differ,maybe the suction effect will differ.
This must be the answer,I guess
Alex