Kraken should do a better job of labelling the different types of orders, but you appear to have done a "stop loss" order which is considered to be a market order. The problem is that market orders are subject to "market price protection" to protect you from really bad slippage. See more here https://support.kraken.com/hc/en-us/articles/201648183-What-is-Market-Price-Protection-

For your purposes, a stop loss limit order would have worked. https://support.kraken.com/hc/en-us/articles/203325793-Stop-Loss-Orders