jerry,
i have the job of enforcing the code as written without the option of enforcing what others may feel are "good ideas" or what is safer.code changes like requiring everything to be retrofitted are good ideas but public outrage would be a bitch to say the least. enter the lawyers and expert witnesses and then a code change.
the 4 3/8" is to allow for equal spacing of balusters on a standard stair run. 2 balusters per tread, or the extra 3/8 for kids with bigger heads to be able to enjoy putting their heads thru.