On OS X, /var/log/system.log is rotated by the
newsyslog command, which is run at half-past each hour (see /System/Library/LaunchDaemons/com.apple.newsyslog.plist) whenever the Mac is running and awake. The rotation rules are in /etc/newsyslog.conf, from which the relevant lines are:
# logfilename [owner:group] mode count size when flags [/pid_file] [sig_num]
/var/log/system.log 640 7 * @T00 J
If I understand this right (see the newsyslog.conf man page), the "@T00" in the "when" field means that the log will be rotated if newsyslog runs between midnight and 1am -- i.e. at the 12:30am run. But if the Mac is off or asleep at 12:30, this run won't happen and the log won't get rotated that day, which is probably why you see such variable results.
If you want to change the rotation criteria, feel free to edit /etc/newsyslog.conf; most of the other logs get rotated based on size, and I'm not sure why system.log is different.