Recently, I corrected a sentence for someone so that it read, "What is the difference between a seamstress, a tailor, and a dressmaker?" Later, the person asked why it wouldn't read "What are the differences between..." instead. Other than the former sentence feeling intuitively correct, I didn't have a good explanation. Then it occurred to me that perhaps it has to do with "difference" being countable or uncountable depending on the situation. Which is to say, when used as a general concept about the dissimilarity of two or more things, it's uncountable, but becomes countable when referring to specific differences, as in, "The differences between us became too glaring to ignore." Does it come down to C/U nouns, or is there something else?
Oxford dictionary defines "difference" as:
a point or way in which people or things are dissimilar.
So, "the difference" would be a single way in which two things differ.
However, one might innocently ask if there is a single difference between two things and receive the response that there are actually several ways in which the two differ. One cannot expect the person asking such a question to know how many differences there are.
However, if there is more than two persons or things being compared, the plural differences should be used:
What are the differences between a seamstress, a tailor, and a dressmaker?
Alternatively, you could ask:
How do a seamstress, a tailor, and a dressmaker differ?
The latter does not assume that there are multiple differences, as two of the three could be entirely synonymous and perhaps only one difference between the third.