Right, I would be very surprised too if a school fell from 18 to 22 and people stopped coming there. However, some firms are very snobbish and what you would see is that maybe the school then drops to 24 then 27 and the firm still comes there but their students really aren't getting the same looks as they used to when the school was ranked #18. Also, for a school to go down another school has to go up, the large firms base their reputation partially of whom they hire and want people from the most prestigious schools. Finally, I think you're right that this may be a result of internal rankings but this would happen more locally... i.e. how chicago firms perceive midwestern law schools. A lawfirm on the east or west coast may have less familiarity/no real internal ranking for a school like indiana so that school moving up and down in the usnews rankings is their only idea of the quality/prestige of the school.