I can only remember the outline details - old age. But there used to be a significant difference between UK registered 744s and US registered ones because of CAA/FAA differences. This may read across to the 737. It all revolves around cruise Center/Centre of Gravity and different certified turbulence max g. The FAA takes an average c of g case whereby the CAA takes actual input into the FMC. In addtion the FAA limit was I believe 1.2 g whereas the CAA equivalent was 1.3g. This results in different max altitude displayed calculations on aircraft Flight Management Computers depending on how they are configured and to which rules.