What does "actually contributed" mean?

Joe implemented feature A. Sandra implemented feature B. Raj implemented C. All launched in July. Since then metric X is up 20%. Who gets credit, and what does that credit really mean?

Now say all 3 did that in 3 different products. One produced a 200% improvement in an internal product, one a 40% improvement in a product with thousands of users, one a 1% improvement in a product with a billion users? Compare *that*.