What is the reason for the difference in iftar and imsak times between two very close border crossings, even though they used to agree on this during the colonial era, and does "differences in horizons" justify this disparity in fasting?

If both countries fast and break their fast based on the legitimate sighting of the crescent moon, then this is an issue of ijtihad (independent reasoning) about which scholars have many different opinions. Some scholars believe that the sighting in one country obligates all other countries (the opinion of the majority), while others believe in considering the difference in moonrise locations or regions. Some also believe that if the sighting is confirmed by the ruler, then his entire populace is obligated to fast.

Scholars have differed across various schools of thought:

1. Each country has its own sighting, supported by the hadith of Ibn Abbas. 2. If the moon is sighted in one town, it obligates the people of all other towns (the well-known view among Malikis), but Ibn Abd al-Barr narrated a consensus against this for distant countries. Ibn al-Majishun said that it only obligates the people of that town unless it is confirmed by the Great Imam. Some Shafi’is said that if the countries are close, the ruling is the same, but if they are distant, there are two views (and the difference in moonrise locations is the correct one). 3. The difference in regions. 4. It obligates every country for which its concealment is inconceivable. 5. Ibn Majishun's aforementioned statement.

The hadith of Ibn Abbas supports the first view: "But we saw it on Saturday night, so we will continue to fast until we complete thirty days or see it." And when Ibn Abbas was asked, "Are you not content with Mu'awiyah's sighting and his fasting?" He replied, "No, this is not what the Messenger of Allah (peace be upon him) commanded us."

This is an issue of ijtihad, and the disagreement about it is ancient. So, whoever finds one opinion to be more preponderant or follows a trustworthy scholar, there is no blame upon him.