You'll find that tilt angles significantly lower than a site's latitude lead to the highest annual energy output calculations in the Solar tool.
The tool doesn't inherently favor a particular tilt angle; it simply calculates the total energy output expected over the course of a year. If there were no atmosphere, the latitude and optimal tilt would match up nicely. However, sunlight has to travel through more of the atmosphere in winter than in summer, and a lot is blocked in the process.
To maximize total energy output in a year, the tilt angle should favor summer more. You might want to use a higher tilt than the overall energy-maximizing one if you want to increase the system's power output during the winter months.
There’s a lot more information on tilt at http://www.solarpaneltilt.com, especially in the “Fixed Tilt” section. This site is not associated with Nearmap.