Guild Wars 2

The art on the front of the heroic edition shows Zojja with 4 fingers per hand, yet in-game asura only have 3.

Which is correct?

Did Zojja invent a gauntlet with an extra mechanical finger?

Or did all in-game asura make the same mistake with the fractal-slicer?

What you see in-game is correct.

Concept art is just that – conceptual artwork. And ArenaNet likes using concept art in their promotions. So the artist (Kekai Kotaki here, I believe) made a mistake and drew a too-human hand. Which wouldn’t be all that unlikely to happen once or twice.