There are a few types of employment data, and they are all collected by survey. Some schools are more aggressive than others with getting students to fill out the surveys. (Clearly, some schools don't *really* want to know the answers to the surveys.) First, people often inflate numbers on the survey to make themselves more impressive, and second, people with high-paying jobs are the ones most likely to respond. I've also read somewhere that for US News purposes, they just calculate that a certain percentage of the people who do not return surveys are employed.
% Employed at graduation: This number is more important in law school than it is in undergrad. If you're not employed at graduation, you may find it difficult to land a job until after you find out your bar results. This means that you're taking out additional loans to pay for your living expenses and bar review course. Also, your loan deferrals will kick in almost in line with when you find out about your bar results. Many students who are employed at graduation, especially with market-paying firms will have all sorts of benefits to get them through the gap. Additionally, some schools hire a few desperate grads as low-paying research assistants in order to boost the numbers.
% Employed after 9 months: These numbers count ANY employment. Your student loan payments have kicked in by this time. Who doesn't have to have SOME kind of employment after 9 months??? Therefore, this number includes people who are working as paralegals, in non-law related jobs, or even retail out of necessity. It also includes people who are working in doc review sweatshops on a contract basis. It's temping for lawyers, and it sucks. You have a law degree and you are doing low-level work, poorly paid, with no benefits and no real opportunity for advancement, etc. But hey, it counts for your school's "employed at 9 months" numbers!
Midlaw Myth: Many students think: "It's OK if I don't get biglaw. I'll just work in midlaw for $30,000 less. $80-100k still sounds great to me! Let the snooty T14 kids fight it out for $130-160k." But if you look at the salary distributions, "Midlaw" is basically a myth. Most of the salaries across the industry are distributed in the high range or the low-end-barely-paying-off-the-loans range. There are mid-size law firms, but most of them pay at the "biglaw" end of the bell curve and are highly competitive. There are lots of T25 grads who WANT secondary markets, mid-sized firms, quality of life, and faster track to partner, so students at lower-ranked schools shouldn't count on the doors to the mostly mythical midlaw opening to them.