What is the ruling on Islamic countries differing in determining the day of Eid, what are the reasons for this difference, and are these reasons due to the اختلاف المطالع (differences in moon sighting horizons) or something else?

The valid Shar'i (Islamic legal) reason for Muslims to differ on the day of Eid is the difference in moon sighting locations (matali'), which is the position held by the verified scholars (muhaqqiqoon) among the people of knowledge. As for anything else, these are political disagreements not approved by the Sharia, and Muslims must abandon their differences and strive for their unity.