I guess I was getting a little too comfortable running my tests with my nifty speedstir and all that. This morning I was out testing my water and suddenly realized I had put 25ml of water in to run CH but I only added enough reagent for the 10ml test, which is my normal test. Dumped it out and tried again. Whaddya know - my CH is 150 lower than my last test... which is what I had hoped to see after draining some water to water the lawn and refilling. And the final blue color wasn't all washed out, either!
It's an easy mistake to make, since I use the 25ml TA test.