It's a little funny the way it works. Dues are based on the Jan-Dec calendar year. But you're actually only paying dues on the points within the calendar year itself.
Example (just to make it easy) you have 240 points with a September use year. Your 2005 dues are payable Jan 2005, or you can do it monthly throughout the year.
For a September UY, the dues you pay Jan 2005, 8/12th of the dues are for your Sept 2004 use year, and 4/12ths are for your Sept 2005 use year.
In other words it's like you took your use year and divided the total points into 12 monthly amounts. For 240 points that would be 20 points a month. For your 2004 use year, eight of those months are actually in calendar year 2005 so technically you're paying dues on 8-months, or 160 points (at 20 points per month) for your 2004 year points. You also have 4-months worth of 2005 points (Sep-Oct-Nov-Dec) which would be 80 points (at 20 points/month).
So your total 2005 dues covers 160 of your 2004 UY points and 80 of your 2005 UY points, or a total of 240 points.
Hope that helps.