Long ago, a good friend of mine told me about some softwares he had bought.
You cannot say “softwares” I told him, it’s “pieces of software”, or some other construct. He did not believe me, but I was right, though in those pre-internet days, it was difficult to prove.
More recently, I edited the Pound Sterling page on Wikipedia because it said “There is a variety of theories”. Again, ‘variety’ is uncountable, so the correct version of the verb “to be” is “are” to agree with the number of theories. There is more than one theory, so it should be “are”.
This is complex English, and not something that might be expected of even an eighteen year-old learner of English as a second language.
But there it is. If someone asks you:
“Is it ‘is a variety’ or ‘are a variety’?”
Then the answer is that “is” and “are” must agree with the noun of which there are a variety.