I agree, that high should be the maximum sustainable output, and “turbo” only burst-mode that’s not meant to be held for long, and like it or not, drop down to high (or lower, if cooking) for something that’s sustainable. If the two levels are too close, I wouldn’t bother, and either step down “turbo” to something sustainable, or step down high if it starts cooking itself.
But often, mfrs will go with the wow factor, ie, step down high so that the jump is dramatic. “Turbo” is simply “brighter than high”, ie, meaningless.