From wordcounter.io, which should know about word counting
How many words per page are there in a typical novel? For manuscripts, the answer is about 250–300 words per page if it is double-spaced.
And that is the average. Thus 340 is not entirely off the chart.
To satisfy your curiosity you can try copy-pasting Ulysses by James Joyce into your setup and checking what's the average number of words per page. I think it goes around 320.
Note that the number of pages may well depend on the average length of your words, whether you have a lot of dialogues, and whether you have short paragraphs, or just a wall of text.
Also, Scrivener has two approaches to computing the number of pages, and one is the 'accurate one', which will actually try to fit your text onto pages with the settings you have provided. Another way to test this would be to generate a PDF, check that the measurements of the page are correct in terms of paper size, margins, font and line spacing, and check the total number of pages.