A B757-200 DEPARTING PHX; MAKES A SUDDEN L TURN AFTER DEPARTING RWY 8R AND THREATENS TO CROSS THE DEP PATH OF ACFT FROM RWY 8L.

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DEPARTING PHX MAR/XA/99 ACFT X AT XX11 ON THE ST JOHN'S 2 (ATTACHED) FOLLOWING VISUALLY BEHIND AN ACR FLT RWY 08R. LNAV WAS ENGAGED WHICH COMMANDED A L TURN OF APPROX 068 DEGS TO TRACK INTO PXR. THE APPARENT DIFFERENCE WAS SPLIT BTWN WHAT I THOUGHT SHOULD BE FLOWN AND THE COMMANDED LNAV TRACK INTO PXR. APPROX 073 DEGS WAS FLOWN TO PXR WHEREUPON PHX TWR ADVISED ME WE WERE NOT FOLLOWING THE ST JOHN'S 2 DEP. THE SUBSEQUENT FREQ CHANGE TO PHX DEP ALSO ADVISED US WE WERE NOT FOLLOWING THE DEP. AT NO TIME IN THE FLT WAS A MAP SHIFT NOTICED OR LNAV NOT PERFORMED PROPERLY. HAVE ANY OTHER CREWS EXPERIENCED AN LNAV COMMANDED L TURN USING THE ST JOHN'S 2 OFF RWY 8R? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SUSPECTED THE DEP RADIALS WERE WRONG. HOWEVER; IN CONVERSING WITH THE RPTR; IT BECAME KNOWN THAT HE WAS UNAWARE THAT THE LNAV FEATURE WOULD BE CALCULATING THE GND TRACK FROM THE LIFTOFF POINT TO THE PXR NAVAID. SINCE HE HAD FLOWN STRAIGHT AHEAD AFTER LIFTOFF; THIS PLACED THE COMPUTED GND TRACK TO HIS L. WHEN LNAV WAS ACTIVATED; IT TOOK A BIG TURN TO THE L TO GET ON COURSE. RPTR WAS INFORMED THAT IF HE WOULD EITHER FLY MANUALLY TO THE VOR OR USE THE DIRECT INTERCEPT FEATURE OF THE LNAV NO SHARP TURN WOULD OCCUR. FURTHER; IT WOULD BE A SMOOTH TURN TO FOLLOW THE PRESCRIBED DEP.