What are the sources of differences in ethics between people, companies, and nations?
For individual people there are as many explanations are there are individuals, and as there are over seven billion people currently alive, it would be impossible to account for all of them.
In terms of groups of people, whether companies or societies, generally cultural and ethical traditions evolve to solve the problem of how people can live and work together in groups. Many different systems work reasonably well, but in an era of increasing globalization we run into problems when different ethical systems run into and clash with each other.
Some societies, often ones that are widely spread out rather than highly urbanized in their formative periods, subscribe to a rugged individualism in which social cooperation happens locally and individually rather than being legislated. These groups often place a high value on individual freedom and are willing to sacrifice collective goods for it.
At the other extreme, some groups, often ones in areas where agriculture requires extensive cooperation, evolve ethical systems based on the good of the community, in which individual needs are subsumed to communal needs. Other groups, often nomadic or village cultures, tend towards tribalism, in which needs of an extended kinship group are considered most important.
Many modern societies have developed general and abstract systems of ethics, emphasizing a rule of law. This notion of abstract law and companies and politics following rule-based systems clashes with cultural systems based on tribalism (in which one is obligated to value helping tribal members over abstract law) or ones with more informal sets of accepted practices. While governments can, for example, pay civil servants very little and accept that bribes will supplement their incomes (with the bribes functioning in many ways in lieu of taxes), other cultures regard bribery as corrupt and instead have a system of collecting taxes and having a larger civil service paid living wages. Either of these systems can work, but they do not work well together.
This is a great question. Many things could be stated, but I would say that one of the greatest sources of differences among people, companies and nations is the level of responsibility. In other words, people have a certain level of responsibility, but companies have more. For example, people have to worry about themselves and their immediate surroundings like close friends and families. When it comes to companies, there is much more at stake.
A company in a sense is responsible for the employees. And if the company is big, then there are many employees. Furthermore, the company may also have social and environmental responsibilities.
When it comes to nations, the stakes are even higher. Countries have responsibilities over their citizens and even international considerations.
In light of all of these points, one of the chief sources of differences comes from size and scope. I will add a few links that will shed further light on this important topic.