Some airlines will only show a maximum of 7, or 4 seats rather than 9. So sometimes if you see a display that looks like F7 A7 J7 D7 Y7 etc (or F4 A4 J4 D4 Y4) it may only be that it's one of those airlines. But if it's J9 C9 Z4 Y9 B9 M9 H9 Q9 V9 then you can be pretty reliably sure that there's only 4 seats available for sale in that class at the moment.