B747-200 CAPT RPTED THE ACFT ALT IN ERROR IN THE POS RPT TO RPMM.

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ENRTE SBOUND A-582. PRIOR TO SAKON; RECEIVED CLRNC TO CLB AND MAINTAIN FL390. LEVELED AT FL390; RPTED FL390 AT SAKON. RECLRED TO FL400 FOR DIRECTION OF FLT ENTERING REDUCED VERT SEPARATION MINIMUMS AIRSPACE. CLBED TO FL400. AFTER LEVELOFF; FO'S ALTIMETER READ 40010 FT. CAPT'S READ 39965 FT. AT BISIG; AND SUBSEQUENTLY AT ELMAS; CAPT PREPARED POS RPTS FOR THOSE FIXES. GLANCING UP AT HIS ALTIMETER; HE SAW 39965 FT INDICATED AND READ IT AS FL390; WHICH WAS ALSO THE SAME ALT AS HIS PRECEDING POS RPT AT SAKON. FL390 THEN WAS WRITTEN IN ON THE FLT PLAN AND THE POS RPTS WERE READ TO MANILA RADIO AS FL390. SEVERAL MINS PASSED BEFORE MANILA RADIO CALLED BACK TO CONFIRM THE ALT. THE CAPT THEN REALIZED THE ERROR AND CLARIFIED TO ATC THAT THE FLT WAS INDEED LEVEL FL400 AND HAD BEEN LEVEL AT FL400 AT BISIG AND ELMAS AND THAT THE FL390 RPT AT THOSE FIXES HAD BEEN IN ERROR. FACTORS CONTRIBUTING TO THE ERROR WERE DISTR DUE TO MEAL SVC IN THE COCKPIT AND THE ELECTRONIC ALTIMETER DISPLAY. UNLIKE MOST ALTIMETERS; THE ELECTRONIC ALTIMETER DOES NOT EMPLOY A ROTATING DRUM IN ITS DIGITAL READOUT. CONSEQUENTLY; THE THOUSANDS DISPLAY AT 39990 FT LOOKS IDENTICAL TO THE THOUSANDS DISPLAY AT 39000 FT. BOTH ELECTRONIC AND ROTATING DRUM TYPE ALTIMETERS ARE USED WITHIN THE FLEET. I HAVE ENCOUNTERED THIS TYPE OF 'READ ERROR' SEVERAL TIMES BEFORE THIS OCCURRENCE.