In short, it’s because they used different calculations to find the first year of the calendar. From Wikipedia:
> To indicate the year, followers of the Ethiopian and Eritrean churches today use the Incarnation Era, which dates from the Annunciation or Incarnation of Jesus on 25 March AD 9 (Julian), as calculated by Annianus of Alexandria c. 400; thus its first civil year began seven months earlier on 29 August AD 8. Meanwhile, Europeans eventually adopted the calculations made by Dionysius Exiguus in AD 525 instead, which placed the Annunciation eight years earlier than had Annianus. This causes the Ethiopian year number to be eight years less than the Gregorian year number from January 1 until 10 or 11 September, then seven years less for the remainder of the Gregorian year.
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