I was looking at Beethoven's sonatas and there seem to be 2 sonatas under Quasi Una Fantasia.

Those are Sonata no 13 and the moonlight sonata.

I thought Quasi Una Fantasia was just Sonata no 13.

But I seem to be wrong.

The same name for 2 sonatas is confusing. I mean with sonatas, if you are going to name them as well as number them you should have a different name for each sonata.

So Why does Quasi Una Fantasia refer both to Sonata no 13 and the moonlight sonata?