Yep thanks for adding this clarification! Planes swap out last minute for operational reasons all the time, so nothing is guaranteed until you're physically on the plane.
Kayak.com added this because it is a meta-search, but they left it off their other direct booking sites (booking.com and priceline.com) I suspect due to liability reasons. This is why almost all car rental listings say "Honda Civic (or similar)" because the person taking your money does not control the car you end up with.