No matter how accurate or inaccurate typical titration test kits are, even the most accurate will have bad batches. Their are other "variables" that can come into play, as Estanoche mentioned. The change in color can also be perceived, differently, by different people.
As an example of one 'variable,' I've always used Salifert. With the Calcium test kit, I was always a bit confused by the directions, and the wording "...clear blue..." The words "clear blue" can be interpreted differently. With the Salifert Clacium test kit, the color changes to a "cloudy blue" and then a "clear blue." So, do the instructions mean that you've reached your end point when the color has changed to a "clear blue," or when the color has clearly changed to blue, even though it's cloudy? I happen to know the answer, but ONLY because I contacted Salifert for "clarification," which led to a thread on RC about this. Most people, including myself, believed that they'd reached the end point, when the color changed to a "clear (non cloudy) blue." This is NOT the case. The end point has been reached when the color has clearly changed from purple to blue, YES the cloudy, but clearly, blue.
So, interpretations can cause slight inaccuracies. The way our eyes interpret the color, can cause slight inaccuracies. Even the lighting, in the room, can effect the color that we see, again, causing slight inaccuracies.
None of this even takes batches into account.
Another thing that can effect accuracy is the expiration date AND how old the kit is, regardless of the expiration date. As the liquids are used up, any slight inaccuracy will be compounded. For instance, take a salinity calibration fluid... Each time you open it, there's a miniscule amount of evaporation possible. The longer you leave the lid off, the more evaporation happens. As the fluid is used up, any possible evaporation is magnified, making the fluid even less accurate, with time. The same holds true for any titration test kit.
. Take Instant ocean as an example. Every poll or thread I've ever seen conducted on which salt brand is the best, instant ocean always seemed to come out on top over the others mainly due to their consistency. I always use instant ocean and would never switch, but on that same note I have had bad batches of it I had to toss. You buy an expensive powerhead like a vortech and you will get one or two out of the box that has a problem. It's just the way it goes. Nothing is perfect. You may have gotten a bad kit which could have happened had you bought salifert instead. Nothing is a guarantee. Then on that same token, maybe the test was right (unless you confirmed the inaccuracy with a different kit) and the new water may have been bad/high in ammonia. I use to buy my ro/di water from a water plant here and had to switch using that water company's water when they started producing inconsistent water. People were getting sick even and the first thing people jumped up and said, "nope! It can't be the water! That's filtered water" until tests were performed. Now hardly anyone uses them anymore and buys their water from the other water plants instead and this was a major water plant here. 
