A B757-200 DEPARTING SEA ON THE MOUNT 5 DEP DEVIATED FROM THE PUBLISHED TRACK WHEN THE FMC COMMANDED AN EARLY TURN OVER A FIX.

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DURING N DEP FROM SEA; ACFT COMMITTED TO EARLY TURN APPROX 1.5 NM PRIOR TO DESIGNATED FIX. ATC RPTED NUMEROUS ACFT HAD EXECUTED SIMILAR MANEUVER THAT MORNING AND ASKED WHY THE EARLY TURN. DURING PREFLT OF FMS; IT WAS NOTED THAT IN SPITE OF A RWY ENTRY OF RWY 34R; THE MOUNT 5 DEP WOULD DISPLAY A SOUTHERLY DEP TO NICHY WITH AN E VECTOR OVER NICHY. THE CORRECT DEP SHOULD HAVE DISPLAYED A NORTHERLY DEP OVER NEZBUG AND THEN AN E VECTOR. TO CORRECT THE ERROR MANUALLY REQUIRED ENTERING SEA TO NEZBUG ON THE LEGS PAGE AND DELETING THE SOUTHERLY ROUTING. THE 'GOTCHA' IS THAT THE EASTERLY TURN AT NEZBUG TO A HDG OF 070 DEGS; IF DISPLAYED ON THE FMC; RESULTS IN AN ANTICIPATORY LEAD TURN TO THE EASTERLY HDG AND DOES NOT ALLOW FOR AN 'OVERFLY' OF NEZBUG. SINCE THIS IS NOT AN RNAV DEP AND HAD TO BE ENTERED MANUALLY; THE ERROR WASN'T RECOGNIZED UNTIL THE ACFT HAD ALREADY STARTED THE EASTERLY TURN. THE TURN WAS SHALLOWED BUT APPARENTLY IT HAD ALREADY BEEN OBSERVED BY ATC. AFTER HAVING CONFIRMED THE SOFTWARE ERROR BY RE-ENTERING THE DEP IN RTE #2 OF THE FMC; DISPATCH WAS ADVISED AND IT WAS RECOMMENDED TO DISSEMINATE THE INFO ASAP. FURTHER DISCUSSION WITH ATC VIA LANDLINE ALSO SUGGESTED THAT NO ACTION WAS PENDING AND THAT THEY APPRECIATED THE EXPLANATION.