There is only a very small relationship between SWG and AWG, but none of the gages is is manufactured by industry.
E.g. to find out the equivalent value of 25 SWG wire in mm the table contains numbers for SWG 24 and 26. Using the arithmetic average of 0.559mm (SWG 24) and 0.457mm (SWG 26) this would mean a wire diameter of 0.508mm.
Who produces that wire?
Going by the WG numbers the only thing one can imagine, that a low number indicates a big diameter decreasing with increasing WG number.
Also there is a sudden stop at SWG 50 which equals 0.0254mm. I have worked with wire of 0.006mm diameter (thinner than human hair). What is the equivalent SWG number for it?
Not surprising it doesn't have an equivalent number, it's just not practical when you get that small. At that point you should be specifying the circular diameter or cross sectional area of the wire explicitly in it's native measure. AWG SWG etc.. are only basically useful for common household/electronic wiring. Once you get really small or really large the standards are only guidelines as you need to take into account manufacturing tolerances.