What is the difference between the Christian and the Jewish calculation of Anno Mundi?

Both Judaism and James Ussher (and the premillennialists afterwards) attempted to date the year that the earth was created ([Anno Mundi](https://en.wikipedia.org/wiki/Anno_Mundi)) . But Judaism set Anno Mundi to be [3761 BC](https://en.wikipedia.org/wiki/Anno_Mundi#Jewish_tradition) while James Ussher set it to [4004 BC](https://en.wikipedia.org/wiki/Ussher_chronology) . (Although Jewish calendar month is lunar, they try to [align the length of the year](https://www.myjewishlearning.com/article/jewish-calendar-solar-and-lunar/) to match solar year, so the year number should be roughly comparable.) **Question**: What's the difference in the calculation method which yield different year number for the creation of the world?