launchd with StartCalendarInterval inconsistencies

I have auditing turned on full (flags: all) on two machines: my Mac Pro workstation and a headless Mac Mini I will be using as a web server. Both are running the latest Snow Leopard. I created a script to rotate the audit logs, and I created a launchd configuration file to run this script one minute after midnight. The launchd plist file is in /Library/LaunchDaemons. Both machines are set not to go to sleep.

On the workstation it fired off fine: at 1 minute after midnight.
On the Mac Mini it didn't fire off until 5:24 am.

Does anyone know why launchd runs the script at the correct time on the Mac Pro, whereas the Mac Mini doesn't get around to launching it over 5 hours late?

Here is the launchd plist file:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>com.netsq.RotateAuditLogs</string>
<key>ProgramArguments</key>
<array>
<string>/etc/security/RotateAuditLogs</string>
</array>
<key>StartCalendarInterval</key>
<dict>
<key>Hour</key>
<integer>0</integer>
<key>Minute</key>
<integer>1</integer>
</dict>
<key>UserName</key>
<string>root</string>
</dict>
</plist>