Why won't kern.maxfiles setting in /etc/sysctl.conf stick?

I've been getting various "too many files open in system"-type errors on Yosemite (10.10.1). Searching around suggests setting kern.maxfiles and kern.maxfilesperprocess in /etc/sysctl.conf.

I've tried this, and kern.maxfilesperprocess sticks after a reboot, but kern.maxfiles doesn't.

% cat /etc/sysctl.conf 
kern.maxfiles=20480 
kern.maxfilesperproc=18000

(reboot)

% sysctl kern.maxfiles
kern.maxfiles: 12288
% sysctl kern.maxfilesperproc   
kern.maxfilesperproc: 18000

If I set it manually at the command line, it works, so it doesn't seem I'm exceeding any upper limit:

% sudo sysctl -w kern.maxfiles=20480 
kern.maxfiles: 12288 -> 20480
% sysctl kern.maxfiles               
kern.maxfiles: 20480

I have seen warnings about these values being overridden in /etc/launchd.conf but I don't have this file:

% cat /etc/launchd.conf
cat: /etc/launchd.conf: No such file or directory

Spotlight search doesn't find any other mention of kern.maxfiles, so I'm out of ideas. Any other suggestions?