Why is the MAHA-UMMAGGA JATAKA numbered #546 in the Cowell book, vol. 3 (of Motital…publication), p. 156 of the original Vol. 6 (I think), but on Sutta Central is numbered #542? See: SuttaCentral. Part of the story relates to DN 5.
Our numbering follows th system of the Mahasangiti Pali edition, so the translation numbers have been adjusted to fit. I haven’t looked into the exact reasons for the variation, but usually it is because one of the editions abbreviates repeated texts.