Colin,
the reason was in the beginning (2004) that the sizes should fit to the Hole Type IQIs of E1025 and it says
"1.3 The values stated in inch-pound units are to be regarded as standard."

As it is nearly impossible to manufacture such small grooves (for reasonable costs) we decided to change the groove sizes to

(as written in my post from 16.11.2019, 02:16)
and now we meet the metric world of ISO standards.

Dear Klaus:
When measuring MTR by using five-groove wedge phantom, how should I accurately determine that if the groove "disappeares in the noise" (9.3.1.2)? For now, I rely just on profiler analysis, but measurement results of the same picture were not consistent (a few millimeters of error).

The intention with the grooves is similar to the wire from the Wire Sets (ASTM or EN/ISO) and the five groove wedge was made for users who do not want to use too much technique when doing the evaluation.
With the wire you say: "I see it" or "I don't see it" and here the visual interpretation was in our minds when we created this IQI. A deviation of a few millimeters should not be an issue as noise is mainly the limit ...