Just a quick question :
Why do some long-range aircraft (eg. transatlantic flights) such as the 757-200, 767-200, 777-200, A330...etc. have a main cruising altitude which is initially sometimes as high as 40,000 feet while some aircraft such as the 767-400ER, 757-300...etc. sometimes never reach an altitude of even 32,000 feet?
The other day I noticed an AA 777-200ER from LHR to JFK reach 40,000 feet just 30 minutes after takeoff!
I assume the difference has to do with engine performance but can someone confirm?