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It looks to me like the frame builber has cut the fork blades too short so he has filed off the necessary clearance from the underside of the fork crown. No allowance for any grit that may be picked up by the tyre, fitting of different size tyres or for any flexing of the wheel. I would want at least a couple of millimetres clearance. For me this is needs to go into anti.

On boards there wouldn't be be any grit or need for other size tires than those it was built for.
Deservedly here.
I would be hesitant to ride those kind of clearances on a curved-blade fork on the street, though!