Bugzilla optimizes database lookups by storing all relatively static information in the versioncache file, located in the data/ subdirectory under your installation directory.
If you make a change to the structural data in your database (the versions table for example), or to the "constants" encoded in defparams.pl, you will need to remove the cached content from the data directory (by doing a rm data/versioncache), or your changes won't show up.
versioncache gets regenerated automatically whenever it's more than an hour old, so Bugzilla will eventually notice your changes by itself, but generally you want it to notice right away, so that you can test things.