if the rim is too narrow then the tread won't have a flat footprint, I mean it will because of the weight of the vehicle on it, but at the expense of tire wear. for example if the rim is too narrow then the tire will wear more in the center at it's proper inflation... to have a even tire footprint on a rim that's too narrow it will need less air than recommended.
when the manufacturer recommends a rim range they are saying a range that will still provide even footprint pressure across the tread at the recommended air pressures... it's not that it won't work or won't fit, it's that it will not work as it's designed to work & Will cause uneven wear so with that setup you can expect to see the center of the tire wear more than the sides... how extreme is hard to say, every tire manufacturer uses different methods & specs to determine the suitable range that they consider "in spec"... real world I guess it's not a huge deal, but it is something that most don't realize...