Is it possible pool store adds stabilizer to sodium hypochlorite?

I'm trying to figure out how I ended up with high stabilizer levels this year. I am getting a reading more than twice what I should be getting for the level of cyanuric I added in spring. I use only sodium or calcium hypochlorite, generally 10% sodium from the pool store.

One possibility is that I had false readings in the cool spring weather/cold water.

I am also wondering, though, whether it is possible that the pool store adds some cyanuric to the sodium hypochlorite tank. Has anyone ever heard of something like this or am I being silly? Could I dilute the chlorine to a certain level to reliably test it straight from the bottle? I wouldn't think testing it undiluted would be possible.

I don't think they can do that.