There's a better picture if you search for it but here you get teh idea...the shape of the Y helps to even the output by the geometry...at least an ATTEMPT to have equal static pressure in teh muffler.
It's not a big deal...but its still a hack job. I used to see it all the time when I dealt with ducts and lazy, imcompetent, and inconsiderate contractors. When one room gets too much air and another not enough...it becomes a bigger deal i guess.
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