MJ probably had the potential to run in 42.6. Another way to look at it is for 400 meter, you double up your 200 meter time and add 3.5 seconds (for elite athletes). For high school athletes you might need to add anywhere 4 to 6 seconds depending on their special endurance. With MJ 19.3 x 2 = 38.6 + 3.5 = 42.1. So could be couple of reasons. When MJ ran his 400 WR, probably he was in 19.8 shape (for 200) rathen than 19.3 or may be his special endurance was not as good as other top athletes. But the doubling of 200m time and adding 3.5 to it has worked out close to figuring out 400 m best (or potential best) for elite athletes. MJ also set the record in World championships and he had to run couple of rounds prior to the final. Though for MJ, probably there were easy runs, nevertheless he had to run the rounds.