I have heard it said that some manufaturers include "Diversity" in the design of the consumer unit. I believe this is typically 80%. This means that although the unit is rated at, say, 100 amps the maximum load is assumed to be unlikely to rise above 80 amps for any appreciable length of time. One reason for keeping the larger circuit breakers closer to the switch is that, on the occasions the total load is at or above it's 80% maximum, the length of bus-bar conductor under full or overload condition is at its shortest and therefore less likely to cause overheating to the other circuit breakers.
Steve, November 2009