First, there was a simple 'Week view' for which you could set an action. If you set a day on an action, it will be shown on that day in the Week plan. If you don't, it would be shown on the 'Not set' section. The quadrants filter filter by this day - it checks for exact value of this property of an action set.Then there came the recurrences - from simple ones like once a week to a little more complex, like several weekdays. This new functionality has to be merged with the previous 'day' selection. And it took priority over the weekday set. I.e., if you have both, the recurrence will be taken into account on the Weekplan page - the action will be shown for each recurring day, and ignore the day set. The problem here is - the filter on the FTF/Actions pages still works with the old day setting. This is definitely a bug (or, rather a not updated functionality), so we'll fix it soon.
Also, there's a third feature that is related - reminders. Up until recently we had only one-shoot reminders. Recently we've added the reminder type 'On recurrence days'. From its name you could have guessed that it fires on the days the action is to be shown. Every day it takes today's actions and schedules reminders for those that have a reminder set. And by 'today's actions' I mean those that are shown on the Week plan by the logic described above.
Again, since the system has been evolving, not everything finished - we are working on that. Hope the logic is clearer and more helpful now.