If on the POMO days, the fed is essentially buying it's own treasuries (buying its own debt), why does the dollar always drop? I can understand the printing money to buy their own treasuries but how does the FX "system" account for extra dollars in existence? If they're buying treasuries with existing dollars, then the dollar shouldn't drop at all. If it was other countries buying then you'd expect the dollar to rise.
http://www.newyorkfed.org/markets/tot_operation_schedule.html
http://www.newyorkfed.org/markets/tot_operation_schedule.html