It's the same amount off on both: 10%.
However, on both stores, if you own an existing copy of one of our games, then you get a loyalty coupon over the next little while that offers you an additional 15% off. Since you're logged in, you see the 25% on steam. On humble it applies it at checkout, I believe. Or they email it to you. I'm honestly not sure; the systems are both new to us in terms of that sort of coupon. But if you look at steam in incognito mode, or logged out, you'll see just a regular old 10%.