The NAK have the correct clearance to slip over the dust seal on the axle (that piece of flanged metal that the seal goes over), the aftermarket brands are too small and fail because of friction .
The only bit of the seal meant to be touching the axle is the lip with the spring around it. When you have the correct seal clearance the axle slips in easily, when you have to push it the last inch the seal is too tight.
My experience only, others may differ. There is a good deal of info floating around with diagrams and measurements but the easy peasy answer is always get the genuine NAK brand.
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