There are a lot of different words for many concepts in classical Chinese, for instance for the personal pronouns. Due to which circumstances could this happen?
Synonyms arise when two different topolects or languages are mixed, perhaps as a result of warfare or a political union, but also because of technical innovation, cultural exchange and so on. This happens constantly in all languages.
As for pronouns in classical Chinese, they correspond to a highly hierarchical social order. You would often need to refer to yourself in third person to show respect and to be in line with the Confucian ideal of humility. Even the emperor could refer to himself as 寡人, third person for one of a few select.
Then again, if the emperor calls himself 朕 (I), then everyone below him would have to refer to the self using a lesser term, in hierchical order. 在下 is the one beneath the emperor, 我 is the worthless body, 奴才 is the slave, etc.
There is a thing called 面子 in our Chinese culture. We talk about ourselves in one form of pronouns to show others that "I am humble/modest." or "I respect you a lot." Example: 在下、末将 = I.
But if we got home, when we talk to our family, we are less restricted and we want to show our rights and power to our sons and daughters, so we use another form of pronouns.Example: 老夫、吾 = I.
I hope these will help you.