Keep in mind that not every month is used for use years. There are no such thing as a January, May, July, or November use year.
Also, not all use years are created equal. You might think that points are distributed equally between the eight different use years, but they aren't. Each resort is slightly different, but in general at WDW there are some use years that are more heavily used than others. For example, December and February use years have the most points with 17.6% and 16.8% of the total points, respectively. April and October have the least amount of points with 8.0% and 8.3%, respectively. The other months fall somewhere between. So that means there are twice as many points available with December and February use years as there are for April or October.
Bottom line, there are some months you can't request because they don't exist. Other months are more or less likely to be available in the inventory DVD has for sale.
I'll also make note of a point I haven't seen in this thread yet. Don't sweat your use year too much. Even if you find the perfect use year for how you plan to vacation, today, preferences and life situations change over time. You'll have your contract for a long, long time. The "perfect" use year for you will probably change multiple times while you own the contract. Young couple, Family with pre-school kids, Family with young kids, Family with older kids, Empty Nesters, Retirees... your vacation preferences will probably change at major phases like these. Don't walk away from a good deal just because you can't find the exact use year you had in mind.