MLS Playoff scheduling

Seattle won the Supporters Shield. Why then, does Seattle's opponent get to play their first playoff match a day before DC's opponent does? DC's opponent will go to DC three days after their last game. Seattle's opponent will come to Seattle four days after their last game.

Granted, neither team gets very much rest between matches, but on principle, shouldn't the Supporter Shield winner get all the advantages? What did DC do to get that slight edge that Seattle didn't?

Two separate conferences. My suspicion is these things were scheduled before the season