meaning of the modal verb "would" in context

Question

Here it goes:

Some states have a higher number of people with lower incomes. Why would that be?

Why did not the speaker say "Why is that?"? Does would mean tend there? For example:

Why do some states tend to have people with lower incomes?

Answer 1

Any of the following would be essentially equivalent:

Some states have a higher number of people with lower incomes. Why would that be?

Some states have a higher number of people with lower incomes. Why is that?

Why do some states tend to have a higher number of people with lower incomes?

Omitting the phrase "a higher number of" does change the meaning, since the question becomes why some states have any people with lower incomes, rather than why some states have more people with lower incomes.

Even though these sentences are equivalent, I wouldn't say that the word "would" means "tend". Rather, they both serve to soften the sentence (slightly).

Answer 2

When phrased as "... lower incomes. Why is that?" the speaker is asking what is the exact reason for the lower incomes, however when phrased as "... lower incomes. Why would that be?" the speaker is asking what possible [hypothetical and multiple] causes there might be for the lower incomes.

A British English speaker would also interpret "Why would that be?" to perhaps indicate a level of false naïvety, especially with a stress on "that" eg: "Fred's father runs the company, and Fred always gets picked for promotion. Why would that be?"