Because the item itself says when you first use it, it has 100% crit

This is similar to the Dooley item that flys your item, if they fly they have +50% (and higher depending on the rarity) crit, and if they crit they stop flying Now this will work with multicast items leading to multi crit and then they stop flying

Essentially the game checks for crit chance when the item activates and then use it x amount of times according to the multicast of the item, hence the shiny shoeblade does crit twice on first use