B737-700 CREW FOUND THE ENG START LEVERS OUT OF THE RUN DETENT AFTER DEPARTING PHX.

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START LEVERS OUT OF DETENT AFTER TKOF. PUSHBACK AND CHKLIST USE NORMAL. AFTER TKOF AND DURING THE SJN 2 DEP FROM RWY 25R IN PHX; WE BEGAN A TURN TO AN ASSIGNED HDG OF ABOUT 070 DEGS. THERE WERE SOME TSTMS AHEAD OF US ABOUT 60 MI ON THE DEP. I LOOKED DOWN TO TURN ON THE RADAR. WHEN I DID; I NOTICED THAT BOTH START LEVERS WERE OUT OF THE DETENT AND HAD MOVED DOWN APPROX 1 INCH TOWARDS CUTOFF. I DREW THE FO'S ATTN TO THE LEVERS AND PUSHED THEM BACK UP INTO DETENT. WE HAVE NO IDEA HOW THE START LEVERS GOT OUT OF THE DETENT POS. I PHYSICALLY AND VISUALLY CHKED THEM IN DETENT DURING ENG START. THEY ARE AGAIN PHYSICALLY AND VISUALLY CHKED DURING THE CHKLIST BY ME AND VERIFIED BY THE FO. THERE IS NO DOUBT IN MY OR MY FO'S MIND THAT THE START LEVERS WERE IN DETENT. WE ARE 100% POSITIVE THEY WERE IN THE CORRECT POS. WE TALKED ABOUT THE SIT; AND NEITHER ONE OF US WERE NEAR THE START LEVERS IN ANY WAY OTHER THAN ME ADVANCING THE THROTTLE FOR TKOF AND HE SETTING TKOF THRUST. AFTER THE START LEVERS WERE RETURNED TO DETENT; THEY STAYED THERE AND DID NOT MOVE (AS THEY ARE SUPPOSED TO WORK). ONCE AGAIN; WE HAVE NO IDEA HOW THEY CAME TO BE OUT OF DETENT. THERE MIGHT BE SOME PROB YET NOT NOTED IN THE B737-700. SUPPLEMENTAL INFO FROM ACN 555708: FIRST DAY OF 3-DAY TRIP. ON TIME PUSH. READ THE PRE-TKOF CHKLIST IN ACCORDANCE WITH PROCS PRIOR TO TAXI. DURING PRE-TKOF CHKLIST; CAPT PHYSICALLY CHKED START LEVERS IN DETENT AND I VISUALLY CONFIRMED. TKOF WAS MAX -N1; AS WE WERE HVY AND WE HAD A LONG LEG. AFTER TKOF ON ST JOHNS 2 DEP; CAPT LOOKED DOWN TO WX RADAR AND NOTICED BOTH START LEVERS APPROX 1 INCH OUT OF DETENT. CAPT DREW MY ATTN TO THE START LEVERS AND HE IMMEDIATELY RETURNED THEM TO THE DETENT POS. NO FURTHER PROBS DUE TO THE EVENT. WE DISCUSSED HOW THE START LEVERS COULD HAVE COME OUT OF DETENT AND CAME UP WITH SOME IDEAS; BUT HAVE NO CURRENT EXPLANATION BEYOND A MECHANICAL/HARDWARE THEORY.