It is possible for an order to partially execute (or "fill") if there are insufficient balances in the account for the order to fully execute.
This most commonly occurs when an order is placed for maximum volume. For example, selling 100 XRP while owning 100 XRP.
When trading with your entire balance of an asset, there will likely not be enough funds for both the order and the trading fee. Instead of preventing the order being placed, we allow the order to execute almost entirely, using some of the volume to pay for the fee.
You can see how much of the order successfully executed by looking at the Volume Exec'd column. This should also be reflected in your Balances, visible by selecting the Kraken logo in the upper-left corner.
If you click on the order ID under Trade > Orders, more information will be provided. You can see the reason for a canceled order on that page.