HDX differences between doc and books backgrounds

[stumped]

i have a new HDX 8.9 on which i Have some amazon books and some send-to-kindle books - that latter appear as documents but open in the book reader.

I use the tablet to read in landscape 2 column mode, green theme background

when I open a bought-from-amazon book, I see 2 columns of black text on a solid slab of green background
but when i open a a document e.g. a sent-to-kindle " book", that green background has a subtle shading which creates a 2 page effect with a faint centre seam, like an actual folded out book.

i actually prefer the " document " appearance rather than the "book" appearance, but it seems that I have no say in that - the device serves up what it deems best.

does anyone have any insight into how this works and how it can be worked around. I suppose I could reroute an amazon book via kindle for PC, de-drm with calibre plugin, convert to mobi, send-to-kindle .. to have it as a mobi format document but that seems a ridiculous palaver to have to go through just to force an alternative background effect.
I cannot see anything in the reader settings, - just the standard kindle options for text size, font, background colour etc...

i could try for screenshots if this makes no sense to other HDX owners

[stumped]

screenshot attached ( hopefully) showing the 2 page effect background, on a sent-to-kindle "book" - which is stored as goobledygook_pdoc.prc on the device

what i want to do is achieve this effect on actual purchased books.

I will now send this same book as azw via calibre & calibre companion to confirm that, when loaded as a book, I get only the plain "book" background. OOPS - that is NOT the case - I still get the 2 page effect - very puzzling

so let me attach a screenshot of an actual bought from amazon book to show the difference. and every " bought from amazon" book that I have checked has the plain background also.

I am now truly puzzled as to exactly what triggers the two different background options