I don't know. I've often wondered why they came up with such an awkward specification.
It's there in the 13th and 14th editions (1955, 1966) of the IEE Regs., and in the revised metric 14th edition (1970), but the 4% tolerance is specified in the 15th (1981) edition. It may have actually changed during the seventies as an amendment to the 14th ed.
