the pH of tap water should not be that high. I think 9.5 is the highest that the EPA allows.
And yes, the drop kits have an inherent error which is particularly noticeable when you need only a few drops. You could increase the precision by taking twice the sample volume and dividing the drop count by two. But that doesn't get rid of the systematic error caused by drop sizes that are not the same size as intended by the manufacturer.