Clicking all settings to "on" per the options, the final score is less than 405?

You must be overlooking something or you have not activated ALL features.
Check each section and you’ll probably find you have some features which have a 0 score. (see the score badge in each feature section)

Having said that, trying to rack up points for the sake of reaching the maximum is not the most responsible way to use this plugin because you should always be mindful of whether a particular feature is compatible with your site (especially the “intermediate” and “advanced” features).
So careful monitoring is encouraged when you activate one of the advanced features.