Yup. It's a standard. A misguided one. A rim does NOT have a
measurement which corresponds to ERD until it is known what nipples
are to be used AND how far into those nipples the spoke should
extend. Short of the nipple slot? Flush with the bottom of the nipple
slot? Flush with the top of the nipple? Those are some of the
possibilities
ERD eliminates being able to take these into account with ACTUAL
measurements except by adjusting on the next go-round with the same
components.
Too late now, but a much better figure for manufacturers to provide
would have been ACTUAL ID of a rim. The plethora of existing spoke
calculators would far no worse by also requiring an ACTUAL figure for
the precise desired depth the spoke is to extend into the rim.
DR