GB/T 9113.4-2000

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Does any veteran know which foreign standard the national flange standard from 2000 was based on? It shouldn’t be EN1092, because I found that the pressure rating specified in EN1092.1-2002 is only PN100, whereas the national flange standard from 2000 allows for a pressure rating of PN420. Moreover, why do the pressure grades for flat and raised face flanges specified in the national standard GB/T 9113.1 from the year 2000 include both sequences of pressure grades – PN0.6, PN1.0, PN1.6, PN2.5, PN4.0, PN6.3, PN10.0, PN16.0 – as well as PN2.0, PN5.0, PN11.0, PN15.0, PN26.0, PN42.0? In the national standard flange specification GB/T 9113-2010 for 2010, the pressure grades are PN2.5, PN6, PN10, PN16, PN25, PN40, PN63, PN100, PN160, PN250, PN320, PN400. I know that these pressure grades are derived from EN1092-1-2007. I would appreciate some guidance from experts, thank you!