9mm hub axles don't fit 9mm dropouts?

[DWF]

I'm having a small issue that I have not encountered before. On my '96 Bianchi with a Columbus Genius fork with 9mm dropouts, my two Campagnolo hubs from the same era fit and swap easily. In fact you can see clearance in the photos (first two hubs in photos). I wanted to drop in a different front wheel to experiment with tire clearance and ran into a problem fitting a White Industries Mi5 9mm front hub (last hub in photos) and a Campagnolo hub holding Zonda rims. Both axles got stuck less than 1/4 of the way into the dropout. I don't have a great caliper to measure the opening, but an eyeball on a mm ruler looks like 8.8mm

I've never had this issue before swapping out front rims on 9mm dropouts and wondered if this is a common occurrence, that not every front dropout and axle is exactly 9mm.

Of course, I also would like to know how to fix this issue. Would taking a fine tooth flat file carefully onto the inner dropout surface be OK? Or is it better left to a machine shop or framebuilder?

Any help with this is appreciated.