Beckenham Boy
I'm not a professional pilot, but I have a number of hours in a sim with an airline TRE (training capt)
It sounds as if your 744 may have "step climbed" to a more fuel efficient altitude when the weight loss due to fuel burned allowed this. (jet engines consume less fuel as they climb, in simplistic terms.)
As to 36,500 feet, your aircraft would be flying at a "flight level" which is calculated using a barometric pressure of 1015 hectopascals (millibars in old money)
As the real pressure is usually a little different, flight levels do not necessarily coincide with actual feet, e.g. FL350 might be 34,656 feet, so this probably explains that.
WIth a bit of luck, a professional may come along and give you an authorative answer, but int he meantime I hope this helps.