I have seen this phenomenon before and thus tested an image when high intensity was used briefly, waited, then did another burst of high intensity. No change vs doing a single high intensity. If you do a gradual increase in intensity, it acts exactly the same as if you used diffused light throughout, but the reverse produces the same as high intensity.

I think the answer lies in the self-masking effects of a POP system, where that first burst of light creates the mask and you end up with your given contrast based thereon.