It varies. It depends on a few things like what kind of testing they use. For example, my pool store used the BioGard Alex system that uses a strip dipped in the water, but then a computer to read the strip. Test strips, even when used in the store are often inaccurate (mine varied widely). Some stores actually use the drop tests similar to the ones found in the Taylor K-2006 or TF100, but even if they do use those, it's going to depend somewhat on the person doing the test and their experience.
What you're likely to find is that getting a good test kit of your own (K-2006 or TF100) and learning to use it, you will KNOW you have accurate results everytime and be able to do the tests on your schedule.
-- Pete