What type of plants do you have in your tank and have you been testing your tap water's KH? Sometime public water supplies differ in the KH of the water. Mine varies about 1 degree between spring and fall. Depending on which part of the country you live in, your water supplier may use either groundwater or surface water, or both.
Here in Dayton, the city uses Calcium Hydroxide to soften the water, which reduces the Total Alkalinity of my water from 270ppm to 72ppm. This procedure also strips quite a bit of calcium and magnesium from the water. I assume this would also allow for some human error and cause a difference in KH of your tap water.
South Western Ohio Aquatic Plant Enthusiasts (SWOAPE) member