CAPT CAUSES ACFT TO ASCEND 280 FT ABOVE ASSIGNED ALT.

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WE WERE OPERATING A BOEING 767-300; CRUISING AT FL310; WHEN WE DECIDED THAT FL350 MIGHT BE A BETTER ALT FOR US. I ENTERED FL350 INTO THE FMC ON THE CRUISE PAGE TO DETERMINE WHAT OUR PERFORMANCE COULD BE AT THAT ALT. FL310 WAS CURRENTLY DISPLAYED IN THE ALT WINDOW ON THE MODE CTL PANEL. AFTER DETERMINING THAT OUR PERFORMANCE WOULD BE ACCEPTABLE; I NEEDED TO ENTER FL310 BACK INTO THE FMC'S CRUISE PAGE; SO IT WOULD CORRESPOND WITH OUR CURRENT CRUISING ALT. I WAS CURIOUS TO FIND OUT IF I SLIGHTLY ADJUSTED THE ALT ON THE MODE CTL PANEL A FEW HUNDRED FT; ABOUT 300-400 FT ABOVE; THEN RESELECTED FL310 IN SECONDS WOULD 31000 FT APPEAR IN THE SCRATCH PAD ON THE FMC'S CRUISE PAGE; WHICH IS A TIME SAVING FEATURE THAT IS DESIGNED INTO THE SYS. ON SOME AIRPLANES OF THE SAME TYPE; A 100 FT CHANGE IN THE MODE CTL PANEL SELECTED ALT WILL TRIGGER AN ALT DISPLAY ON THE FMC'S SCRATCH PAD. ON THIS PARTICULAR ACFT; NO ALT APPEARED ON THE SCRATCH PAD. THE AIRPLANE IMMEDIATELY ENTERED THE CRUISE CLB MODE; ADVANCING THE THROTTLES AND INCREASING PITCH. THIS IS THE QUICKEST RESPONSE I HAVE EVER SEEN TO A CHANGE IN PROGRAMMED ALT. FL310 WAS DISPLAYED IN THE MODE CTL PANEL ALT WINDOW; WE CTLED THE ACFT SMOOTHLY OR I SHOULD SAY; SINCE THE LEG WAS THE COPLT'S; HE CTLED THE ACFT SMOOTHLY; DISCONNECTING THE AUTOPLT AND RETURNING THE ACFT TO FL310. THE ALT GAINED WAS 280 FT. TO PREVENT A RECURRENCE; I WOULD CONSIDER NOT CHANGING THE ALT IN THIS MANNER IN THE MODE CTL PANEL OUT OF CURIOSITY; UNLESS ONE IS CERTAIN OF THE CONSEQUENCES. ONE CURIOUS NOTE. WHY DIDN'T THE ACFT CAPTURE ANY ONE NUMBER OF ALTS IN THE MODE CTL PANEL WINDOW SUCH AS FL311; OR FL312; WHICH THE SYS SHOULD HAVE SENSED WHEN I DIALED FL310 BACK INTO THE MODE CTL PANEL; OF COURSE PASSING THROUGH THESE ALTS FIRST.