Why the Ethiopian Calendar numbering is eight years late in relation to the Gregorian?

from the series of short articles about calendars

Ethiopian Calendar has exactly the same structure with the Coptic Calendar (see also the info item 14 about the Coptic Calendar). It has 12 months of 30 days plus a little month of 5 or 6 days (in the case of the normal or leap year). The start of the year occurs on 29th / 30th of August in the Old Style Julian Calendar, or 11th / 12th in the New Style Gregorian Calendar. The delayed start occurs every 4 years, in the year just before the Gregorian leap year.
As we was in the case of the Coptic Calendar the years are numbered from the accession of Diocletian (284 AD) in honor of the martyrs tortured and killed during the persecution of Manicheans and Christians ordered by Diocletian and Galerius. Ethiopians and followers of the Eritrean churches use instead the Incarnation Era, which dates from the Annunciation or Incarnation of Jesus on March 25, 9 AD (Julian), as calculated by Annianus of Alexandria c. 400; so their first civil year began earlier on 29th of August, 8 AD (Old Style Julian Calendar). Therefore Ethiopian year 1 started on 29th of August 8 AD. The characteristic phrase following the number of the year is Amata Mehrat ("Year of Mercy"). Each Gregorian year corresponds to a couple of consecutive Ethiopian years. The current year 2018 AD is numbered in the Coptic Calendar as 2010/ 2011 Amata Mehrat ("Year of Mercy").