The number that matters is not your list price and not your royalty percentage. It is what remains after Amazon takes its share and the printer takes its cost, and for a long book priced low that remainder can be a few cents.
Two things decide it. Your royalty rate steps up once the list price reaches a threshold that differs by marketplace, and the manufacturing cost is subtracted after that rate is applied, not before.
Worked example, using this calculator on a 200-page black-and-white US paperback that costs 3.40 to print. At a 7.99 list price the lower rate applies and 0.60 a copy is left. At 9.99 the higher rate applies and 2.59 is left. At 14.99 it is 5.59. Two dollars of list price near the threshold changed the payout more than fourfold.
You crossed the list price threshold at which the higher royalty rate applies. The new rate applies to the whole sale, not only the part above the threshold, so the step at that point is large.
The rate is applied to the list price first, and manufacturing cost is subtracted from that. This is why a low-priced long book can end up with almost nothing left.
The lowest list price at which the royalty covers manufacturing exactly. Below it a book cannot be published at that price, because each sale would owe money.