Tin whisker growth testing PBfree reflow/snpb reflow

Hi i am referring Tin whisker report JESD201A class 2 standard from Vishay. i can see at TC -55/85C using pbfree reflow there are 0 growth at 500&1000 cycles. for snpb reflow and the same temperature and same cycle significant growth can be found. my question is as we use lead to avoid whisker growth then why in Pbfree reflow there is no growth.

Please refer the attached reports below.

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