You see changes because IDL is using monte carlo (random, not repeatable) sampling techniques, combined with irradiance caching. IC dramatically decreases render time in exchange for accuracy. The degree of inaccuracy is adjustable (and so is the render time), but it's always there.

If you have tons of CPU power and all day to render, you can try turning off IC and increasing the IDL number of samples, so that the mathematical approximation (you probably call it a render - LOL) converges on the same values every time.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)