Completely different stocks and people in charge of the two. The resort side likely pays for and keeps track of their own magic band offering. The dot com store side is a completely different group who is in charge of never having a shortage. Then you have the parks side that keeps their own stock at the various stores around WDW.
One having stock has nothing to do with another one. They don't go to the Disney Springs store to grab your Goofy band to ship for your Pop Century stay.
That being said having a much more limited window to when you could select your bands would likely help. Right now you get people with November/December stays taking bands prior to September stays.
My best luck for both of my stays were at 7:30am EST. If you have 6/7/8 people you are not likely to get everyone's first choice.
It sucks but honestly for me it was part of the "fun" as it gave me something to do other than just reading this forum every second and watching
youtube videos over and over lol.