LTT OVERSHOT ASSIGNED ALT WHILE ON THE HAMILTON 6 SID.

Narrative

AFTER DEPARTING CYHM ENRTE TO PIT OUR CLRNC WAS TO FLY THE HAMILTON 6 DEP AND MAINTAIN 4000 FT. I RAN THE CLB CHKLIST; CALLED 1000 FT TO GO TO LEVELOFF; AND REPLIED TO ATC TO PROCEED DIRECT ERI. I TOLD THE FO I WAS LEAVING COM #1 TO CALL OUR TIMES INTO COMPANY. WHEN I RETURNED TO COM #1 WE WERE CLBING THROUGH 4500 FT. BEFORE I COULD ASK WHAT OUR NEW ASSIGNED ALT WAS; TORONTO CTR CALLED AND ASKED WHAT ALT WE WERE CLBING TO. I ASKED THE FO WHAT ALT WE WERE ASSIGNED BY ATC; HE REPLIED I THOUGHT WE WERE CLRED DIRECT ERI UP TO 12000 FT. I THEN REPLIED TO ATC THINKING WE WERE ASSIGNED 12000 FT WHILE OFF COM #1; THAT WE WOULD LIKE TO TAKE 12000 FT AS OUR FINAL ALT. ATC SAID OUR CLRNC WAS TO MAINTAIN 4000 FT AS PER THE SID FOR NOISE ABATEMENT; AND THEN ASSIGNED US A NEW ALT OF 6000 FT. THERE WAS NO CONFLICT WITH ANY OTHER ACFT IN THE AREA AND AFTER FURTHER DISCUSSION WITH THE FO I FOUND OUT THAT HE FLIES THE SAME RTE TWICE A DAY AND THE STANDARD ATC DIRECTIONS ARE TO FLY DIRECT ERI AND CLB AND MAINTAIN 12000 FT. THE FLT WAS HIS SECOND TIME THERE IN 3 HRS AND HE THINKS THE CLRNC WAS FOR THE EARLIER FLT. ANOTHER CONTRIBUTING FACTOR WAS THAT I DID NOT CATCH US PASSING 4000 FT AND QUESTION THE FO.