Not every chip is created equally. When you're mass producing anything, there will be a natural distribution of quality. It's been called the ASIC lottery or chip lottery. That one chip on that card can run slightly faster than the rest at the same power envelop. Said another way, given a fixed clock rate, it will consume less power.
This is how Intel is able to sell "low power" versions of their chips that are drop-in replacements as other chips (ie Intel i5 6500t vs 6500). They're able to cherry pick a few chips that run very well and create a market for them.