How are proc chances calculated?

Hi all. I am trying out a DEE Witch Hunter build and I just got Scorpion Sting from the Scorpion devotion. At first I bound it to DEE and it seemed to proc as the tooltip says (25% chance on attack), but I tried it on Shadow Strike (as the build guide suggests) and now it seems to proc every time.

Why is this? How does this work?

Thanks!

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How are proc chances calculated?