[meme]

Ah, that's because if you don't have a Cover specified, then Sigil will mark the first file in your book as the Cover if it has 1 and only 1 image in it.

Does your first file have exactly 1 image in it? If so, you could remove the image, or add a second, or add a file with a cover and mark it as the cover, and it will stop marking the first file as a cover.

Is there any reason why the first file in your book is not the cover? Perhaps with Apple you don't need a cover file with an image and just need an image? If so, I don't think it causes any issue if you create a page for the cover and include the same image - but someone who uses the iPad would know.