Unfortunately, the water tab does not take into account the amount of water you prepare (and enter as an ingredient) but instead defaults to just the amount of water needed for the recipe. The work around for this is to go to the volume tab and add 4 liters to your mash tun dead space value. Now go back to the water tab, adjust the water ingredient to reflect the new value, adjust this new volume for the water salts needed, then go back to the water volume tab and remove the additional 4 liters of dead space you added. After that, you can adjust the water in the recipe to reflect the lesser value (what you actually need) but the salts will be added based upon the extra 4 liters of water in your HLT.
It is an issue in the water tab which should be addressed for those who make up a volume of water which more than covers their actual needs.