KDP needs a minimum inside (gutter) margin that grows with page count, and outside margins of at least about a quarter inch. Bleed adds 0.125 in on the top, bottom and outside edge, and you only need it when images or colour run to the edge of the page.
Why the gutter grows
Thicker books curve more at the spine, so text near the binding needs more room. KDP's table sets the minimum by page count.
Common rejection
Page numbers or headers sitting outside the safe margin are a frequent reason files get flagged. Keep everything inside.
KDP's rules change from time to time. Check KDP's own help pages for the current requirements before you publish.