So let's break down the question a little bit here. There is variability induced from a variety of factors, but I think the one you're implicitly asking for is production variability. A list of things that can vary within a design tolerance are as follows:
- Propellant mass
- Propellant burn rate
- Propellant density
- Geometric dimensions of propellant features
- Diameter of the nozzle throat
- Rate of erosion of the nozzle throat (Yes, the propellant can start to erode the nozzle enough to impact the performance. It's hard to measure on these small motors.)
All of these will have some effect on the performance of your motor. How much depends upon the particular design of the motor you're using.
In addition to these factors, there are also environmental factors that determine the performance of the motor. Most notable is the there is a direct correlation between the temperature of the motor in a pre-fired condition and the burning rate of the propellant. Consistently igniting the propellant in the same manner is the other (Cessaroni motors really shine over AT motors in this aspect).
Add all of this together with the variability in atmospheric conditions and the condition of your rocket and you'll find that they really aren't as consistent as you'd maybe like. The funny thing about it is that if you were to make the motors yourself, there's a good chance they'd be even worse! There is something to be said for commercial mass production.
One of the biggest things that you can do to reduce motor variability is to buy your motors in large batches. If you get motors from the same lot, there's a good chance they were all cast from the same batch of propellant, which means better consistency within that batch. Just make sure you order enough for the whole season!
If you really want to quantify the effect that all of this has on your performance, you need to set up a Monte Carlo analysis and do thousands upon thousands of simulations. Setting up a good Monte Carlo is a combination of good research, good experimental design, attention to detail, and patience. I think what you'll find is that the true variability of a typical TARC rocket is probably in the neighborhood of 20-30 points or more, and that reducing this variability is a possible if somewhat arduous task.