Is this only for class of '09 placement? I'm pretty sure that these stats are bunk if not -- that is, if they cover EVERY clerk from a school regardless of their graduation year. There are plenty of clerks who have been out of school for years.
Also, there are about ~960 clerks on that list. Let's further assume that there are some schools with maybe one clerk not on that list -- so we round the number to 1,000. There are roughly 800 active federal judges with another 450 senior judges. Let's assume that the average active judge has two clerks*. That means there are at least 1,600 clerk positions. The numbers above only cover a little more than half of that. I don't know how a "senior" judge operates, but even assuming that they only have ONE clerk -- that's another 450 right there that are unaccounted for.
* Keep in mind, with two per judge... I'm not counting the four judges that each SCOTUS member has, and the high numbers that circuit courts normally have (upwards of four or five in many cases). Two per judge seems fair. The district court judge I am interning for has four clerks -- two full-time clerks, a deputy clerk, and a special master for a complex issue. Admittedly, she's probably in the minority with four at the district court level.
In sum, these numbers just don't make sense to me. I know two clerks from a single school alone, and I find it hard to believe that I know ~6% of the clerks from that school.
The OP said the list is incomplete, its only tier 1.