1" threaded headsets (and forks/frames) come in two different sizes, JIS and ISO. The JIS crown race is 27.0 mm ID and the ISO is 26.4 mm so apparently you have a JIS race and an ISO fork

ISO is now almost universal but JIS was used on older Japanese bikes. It's JIS that is now the rare bird, not ISO.

I presume your fork takes the ISO size race and the headset you got is JIS so you will have to return the headset and get the correct configuration. As a rule, crown races are specific to a given model headset. They are not a universal item.