HDG DEV - FLC OR ATC?

Narrative

DURING CLBOUT FROM SEATAC ENRTE TO PASC; WA; THE FO WAS FLYING AND I WAS PERFORMING PNF DUTIES. DEP WAS WORKING NUMEROUS ACFT AND WAS EXTREMELY BUSY. SEA DEP CTLR INSTRUCTED US TO EXPEDITE CLB TO 13000 FT FOR TFC. APPROX 5 MINS LATER SEA DEP GAVE US A HDG TO REINTERCEPT THE AIRWAY (085 DEG RADIAL SEA VOR) AND RESUME OWN NAV. SHORTLY AFTER INTERCEPTING THE AIRWAY; SEA DEP INSTRUCTED US TO TURN TO HDG 040 DEGS; CLB TO 15000 FT AND CONTACT ZSE. THE FO; WHO WAS FLYING; BEGAN TO COMPLY WITH THE INSTRUCTIONS. I SWITCHED TO ZSE FREQ AND CALLED SEA INFORMING THE CTLR THAT WE WERE CLBING TO 15000 FT AND TURNING TO HDG 040 DEG ASSIGNED. THE TURN SEEMED FUNNY TO ME; HOWEVER; WE HAD BEEN TURNED ONCE ALREADY FOR TFC. ZSE'S RESPONSE TO THE HDG WAS 'NEGATIVE' AND HE THEN TURNED US BACK TO REINTERCEPT THE AIRWAY. I THEN EXPLAINED THAT SEA DEP GAVE US THE TURN. ZSE CALLED BACK A FEW MINS LATER AND SAID THAT SEA DEP GAVE THE 040 DEG HDG TO A COMPANY FLIGHT #XYZ; OUR FLT WAS #YXZ ; WHO WAS ALSO EBOUND BEING WORKED ON THE SAME DEP FREQ. I KNOW SEA DEP GAVE US; FLT YXZ; THE TURN TO 040 DEGS AND NOT THE COMPANY FLT XYZ; BECAUSE WE WERE GIVEN ZSE FREQ 120.3. COMPANY FLT XYZ WOULD HAVE RECEIVED ZSE FREQ 135.52 FOR DIRECTION OF FLT AND DEST. I BELIEVE THAT THE SEA DEP CTLR WAS EXTREMELY BUSY AT THE TIME AND HE HONESTLY CONFUSED OUR FLIGHT NUMBERS XYZ-YXZ. IN RETROSPECT; I SHOULD HAVE QUESTIONED THE INSTRUCTION TO TURN TO HDG 040 DEGS; ESPECIALLY BECAUSE THE HDG CHANGE WAS GIVEN AND THEN A HDOF TO ZSE WITH NO CLARIFICATION AS TO WHAT THE HDG WAS FOR; TFC; TERRAIN; ETC.