There are a few ways which this can be resolved.
1. Make sure there is a complete seal when the filter is threaded. Ensure the filter is threaded correctly (watch for cross-threading) and that the blue filter gasket is fully seated against the water reservoir.
2. The most accurate way to get reading is to test the water directly inside the pitcher. You can do this by removing the reservoir from the pitcher and sticking the meter in the filtered water sitting in the pitcher.