If you test for water hardness and have a hard water treatment system that does not remove the hardness minerals from the water you may be asking yourself "Why Does Soft Water Test Hard?"
A discussion to answer this question can not be had without reference to the WQA (Water Quality Association) as the WQA is the de facto water treatment industry association and education leader. We will look at how the WQA defines hard water on it's web site. We will also look at the AWWA (American Water Works Association) definition of a softener.
Please note: for the remainder of this article everything in blue italic text below comes directly from the WQA web site.
"Hard water is a common quality of water which contains dissolved compounds of calcium and magnesium and, sometimes, other divalent and trivalent metallic elements. The term hardness was originally applied to waters that were hard to wash in, referring to the soap wasting properties of hard water. Hardness prevents soap from lathering by causing the development of an insoluble curdy precipitate in the water; hardness typically causes the buildup of hardness scale (such as seen in cooking pans). Dissolved calcium and magnesium salts are primarily responsible for most scaling in pipes and water heaters and cause numerous problems in laundry, kitchen, and bath. Hardness is usually expressed in grains per gallon (or ppm) as calcium carbonate equivalent."
The part of the definition to focus on and keep in mind is how the term hardness was originally applied. This is what maters to you and it describes how the water behaves, more specifically how water containing untreated minerals (dissolved calcium and magnesium) behaves.
The WQA definition of hard water goes as follows:
The degree of hardness standard as established by the American Society of Agricultural Engineers (S-339) and the Water Quality Association (WQA) is as follows:
This degree of hardness standard often leads to confusion. Here is why, it is common practice in the water treatment industry to perform a "hardness test" to determine mineral concentration. Nothing wrong with knowing the mineral concentration. The confusion comes in when the mineral concentration measured by the hardness test is equated to the above degree of hardness standard. This standard does not consider how the water behaves.
The test results and the way the minerals behave are simply not equal, they can not be interchanged. What this scale does not consider is water with hardness minerals doesn't automatically mean the water acts or behaves hard, it does not consider how the water behaves. Untreated hard hardness minerals in water will cause the water to behave hard as defined by the WQA.
Whether water behaves soft or hard is observed. If water does not have soap wasting properties, prevent soap from lathering, create an insoluble curdy precipitate, cause buildup of hardness scale or deposit scale in pipes and water heaters and cause numerous problems in the laundry, kitchen and bath then it is not indicating behaviors of hard water. If water is not indicating behaviors of hard water then it is not hard. If the water is not hard it is soft regardless of the concentration of hardness minerals. Simply explained, if water does not behave hard it is soft regardless of the concentration of dissolved compounds of calcium and magnesium shown to be present by way of a industry standard "hardness test".
The reason your soft water tests hard is because you are looking at a test that tells you the concentration of hardness minerals and when testing for the minerals before and after softening with a treatment that does not remove those minerals will tell you the same thing, the concentration of minerals. Hardness test results do not indicate how hardness minerals and your water behaves.