There is more time built in between when you get the e-mail and when the product actually gets restocked and listed as available on their site.
For example...I had an item on a "Wish List" at Midway for the better part of 8mos until last month. I've been signed up for, and re-signed up for, product availability notifications all throughout that time. I happened to be on the site trying to help a buddy of mine locate an item he needed when I looked at the product I've been waiting for which, wouldn't you know it, was listed as IN STOCK!! I stopped what I was doing, immediately logged in and placed an order and it arrived 4 days later. The day after I placed order (roughly 18hrs or so later to be exact)...I got an e-mail notice that the product I wanted was back in stock, but naturally, when I clicked the link...OUT OF STOCK, NO BACKORDER. Had I not coincidentally been on the site and looking when I found it, I'd surely have missed my chance based on getting an e-mail from them. Don't know if there were that many people on the notice list ahead of me that delayed their e-mail (I doubt it as it was NOT an AR-related item, mags, etc.), but for whatever reason, their e-mail notice system doesn't send out notices when the product is relisted on the site...there is a delay of some sort, likely varying in the amount of the delay.