Oops, I'm very late seeing this. Sorry!
It isn't that the test kit results vary from day to day, it is that they are made with organic chemicals, with organic dyes, and those are not stable chemicals. Eventually they deteriorate. So, if your test kit sat on the dealer's shelf for a year before you bought it, or if the dealer allowed his store to get very hot for a few days, like over the weekend, the kits may never be accurate after you buy them. Then, if you are like me, you buy the kit, stash it under the aquarium and when you finally use it, you don't remember how long you have had it, so it may deteriorate on your shelf too. Now, add to that, that these are very cheap kits, so it is highly unlikely that there is good quality control on their manufacture.
Even if none of the above apply to you, just remember that professionals who do testing of any kind always calibrate their test equipment before use, or, at a minimum every few months. And, that calibration is done with an accuracy far better than the test equipment is designed to meet. No one who is paid to test things ever uses anything for measurements unless it is calibrated first and often. Even professional mechanics get their vernier calipers calibrated on a routine schedule.