I have read about the differences of the URI, URN and URL here and here but the answers talk of the differences of the last letter, that is, the differences amongst identifier, name and location respectively.
What I have not understood is why all these terms have the word 'uniform' and what is uniform about them. This Wikipedia section doesn't mention much about the reason why the change was made from 'universal' to 'uniform'.
I would like to find the missing explanation and not just memorize the terms as they are without fully understanding them.
Based on Tim Berners-Lee’s own account, as published in his book Weaving the Web:
At an IETF meeting, Tim Berners-Lee tried to form a working group that would create an Internet standard for what he suggested to be named universal document identifiers.
About the meeting, in his words (page 61):
[…] there was a strong reaction against the "arrogance" of calling something a universal document identifier. How could I be so presumptuous as to define my creation as "universal"? If I wanted the UDI addresses to be standardized, then the name "uniform document identifiers" would certainly suffice.
While he didn’t agree (it was an issue of whether the Web could be something "universal"), there wasn’t much time and so (page 62):
I was willing to compromise so I could get to the technical details. So universal became uniform, and document became resource.
They formed "a uniform resource identifier working group". (And this group then decided that "identifier" wasn’t a good label, and they chose "locator" instead, forming "URL" – which he also didn’t agree with.)
The current URI Internet standard (RFC 3986) describes the meaning of "Uniform", "Resource" and "Identifier" in section 1.1.