Balance means that the sum of the anion (negatively charged ions) charges is equal in magnitude to to the sum of the cation (positively charged ion) charges. These are easy to compute for the cations but a little trickier to do for the anions so most spreadsheet authors don't bother to do the calculation properly instead assuming the bicarbonate charge to be simply the alkalinity divided by 50 which, in a certain region of pH, it is quite close to and to assume that the carbonate charge is 0. It really is not that big a deal to do this correctly so I have come to assume that people have not been able to grasp the chemistry behind the correct calculation. The calculation is spelled out in a sticky on carbonates and bicarbonates in this forum and this forum and in the thread at

https://www.homebrewtalk.com/forum/...ces-basic-math-and-beersmith-question.651939/ we have posted a spreadsheet that implements those formulas if you are interested.

So that's one source of error but there is another. No lab that measures water parameters does so perfectly. The errors in concentrations lead to errors in the computed charges and thus errors in the sums. These contribute to imbalance and, in fact, the magnitude of the imbalance is often used as a quality control check on the lab's work. Let's suppose your water contains some strontium (many water supplies do). Obviously its charge contribute to the cation charge side of the balance sheet and, as the real balance is always 0, its charge is balanced by a like charge from the anion side. But supposing that the lab doesn't assay strontium (most don't)? Clearly this will lead to an increase in the computed imbalance.

In summary, you water report is imbalanced because of

1)Sloppy practice in computing the imbalance

2)Analysis that indicates that there are more of some ions in the sample than there actually are and, at the same time, indicates that there are less of some other ions than there actually are.

3)Analysis that did not measure the concentration of some ion or ions that are actually present.