Why is my build system constantly rebuilding tools/xxx

What triggers a rebuild of tools/xxx? I built an image 12 hours ago, and when I build another now, I am finding that the buildsystem is re-created a many of the tools it built just 12 hour ago. What triggers this?

did you update feeds or do a pull ? Its most likely the tool/s in question have been updated.

I did not, same commit which is why I was confused.