As usual, Dan is spot-on. When I did Fuel Injection 101 at AFP, Don Rivera confirmed that the odd size restrictors for 4 cyl engines (I.e. Anything other than .022 and .028) are based solely on bench flow tests, not orifice dimension. The finish characteristics that Dan mentions - surface finish, edge contour, etc - make significant differences in flow rate. Hence when you buy a, say, .0245 restrictor from AFP, it is really a .0245 equivalent restrictor.