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.
This is because the trading fee was not taken into consideration when placing the order. But during execution, the order will fill as much as possible while accounting for the fee. This will result in an order status of "Canceled" with reason "Out of Funds".
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.