Originally Posted by Balek

Confirming what chillin says. There are 19 SR cards, 1 Alpha SR and 1 Beta SR. So if every SR you get is an alpha, then you reset the box for the next event, and first SR is an alpha again and so on, you should play some lotto dude :P Or maybe you just thought that all SR's in e-gacha are slayers which is simply not correct.

You're missing key information: The box does not reset between events. So, once you've emptied it, your SR is guaranteed to be the event SR.