ALTDEV ALT EXCURSION - PLT ENDS UP 1000 FT HIGHER THAN CLRNC AND IS UNABLE TO EXPLAIN HOW IT HAPPENED.

Narrative

I WAS FLYING THE RETURN LEG OF A ROUND TRIP TO (AUG) FROM (5B2). THE LEG TO AUG WAS FLOWN AT 11000 FT. WAS ON GND AT AUG FOR ABOUT 1 HR. THE RETURN FLT PLAN WAS: AUG; V- 39; NEETS; V-496; GFL; DIRECT 5B2; CRUISING ALT 8000 FT. AFTER CLBING TO 8000 FT; ASKED PORTLAND APCH FOR HIGHER BECAUSE 8000 FT PUT ME IN AND OUT OF CLOUD TOPS. AT XA00 HRS LCL MANCHESTER APCH ASKED ME MY ALT. SINCE I WAS FLYING WBOUND; I ASSUMED THAT I WAS AT 10000 FT. AUTOPLT ALT HOLD WAS ENGAGED AND THE 100 FT OF THE ALTIMETER WAS STRAIGHT UP. MHT SAID THEY SHOWED ME AT 11000 FT. WHY WAS I AT 11000 FT AND HOW AND WHEN DID I GET THERE?? POSSIBLE ANSWERS: -- WHEN I ASKED PORTLAND APCH FOR HIGHER; I ASKED AND RECEIVED 10000 FT BUT CLBED TO 11000 FT ( I HAD JUST FLOWN OUT TO AUG AT 11000 FT). I THEN CRUISED LEVEL AT 11000 FT FOR 3 MINS BEFORE ATC REALIZED I WAS 1000 FT OFF THE ASSIGNED ALT. -- I ASKED FOR AND WAS GIVEN 11000 FT; BUT ATC PUT 10000 FT ON MY FLT STRIP SINCE THAT IS A NORMAL WBOUND ALT. -- I ASKED FOR AND WAS GIVEN 10000 FT; CLBED TO 10000 FT; SET ALT HOLD ON AUTOPLT WHEN AT 10000 FT. THEN THE AUTOPLT DISENGAGED WHILE IN A SLIGHT TRIM UP CONDITION; RESULTING IN A VERY SLOW CLB TO 11000 FT. AS PLANE NEARED 11000 FT; I NOTICED THE AUTOPLT WAS DISENGAGED; SO WITHOUT CAREFULLY CHKING ALTIMETER; I RE- ENGAGED THE AUTOPLT WHEN THE 100 FT HAD WAS STRAIGHT UP. ATC EVENTUALLY NOTICED MY ALT AND ASKED ME TO GO BACK TO 10000 FT. I REALLY CANNOT REMEMBER WHEN OR HOW I GOT TO 11000 FT. MY GUESS IS THAT #1 IS PROBABLY RIGHT. THE PRIOR 2-3 WKS I HAD A LINGERING COLD. ALTHOUGH I FELT FINE THE DAY OF THE FLT; IT IS POSSIBLE THAT I WAS MORE SUSCEPTIBLE TO HYPOXIA. THIS MIGHT HAVE LED ME TO ASKING FOR 10000 FT; BUT THINKING OF 11000 FT? SINCE I HAD ONLY 14 HRS; IN THE PLANE AND WAS STILL LEARNING ABOUT THE AUTOPLT; #3 SEEMS POSSIBLE; BUT I DOUBT IT. TO CONCLUDE; I DON'T KNOW OR WHEN I GOT TO 11000 FT; ATC SEEMS TO HAVE OVERLOOKED THE PROB FOR A WHILE TOO; THERE WERE NO SEPARATION OR TFC CONFLICT PROBS THAT I AM AWARE OF.