I don't think the two pipes do hit the cruciform at 90deg?
Besides which the bends upstream will affect which side of the pipe the pressure is on and where the fastest moving gas is, much like the water in a meandering stream I would imagine.
Then since the pulses are uneven it could lead to the leading pulse setting the pressures in the cruciform for the following pulse to follow. So if one pipe, probably the rear, tended to go one way, it would probably leave a bigger depression behind to draw the other up behind it.
I know I've blown the baffles out of my RH silencer more times than the left and when it does it's always much louder than when the left blows.
