Minimum list price (KDP).

KDP will not let a title be priced below what it costs to print, so it computes a floor by dividing the printing cost by the royalty rate, 50% or 60% depending on where the price lands (KDP paperback printing cost). Printing cost is a fixed amount plus a per-page rate, which on Amazon.com is $1.00 plus $0.012 a page for a regular-trim black-ink paperback over 110 pages, so the floor climbs with every page the manuscript gains. The rate is a step and not a slope: on Amazon.com the 60% band starts at $9.99 and anything at or below $9.98 earns 50% (KDP paperback royalty), so a floor computed at 60% that lands under the threshold falls into the 50% band instead and the real floor is higher than the arithmetic first suggested. This is the number that decides whether a page count is publishable at all, and it is the one authors meet last, after the layout is finished and the interior is set. A book that can only clear its floor above what its category sells for is telling you the page count is wrong, and the fix is back in the typesetting rather than on the pricing screen.