Im sure they just made whatever on the spot. The boxart for smb was probably made by someone at NOA (nintendo america) based on what they saw from the TV. It should be noted that smb sprites were first made on paper in-office in japan, and didn't seem to circulate outside there.
I never noticed how off the sprite-work on the SuperMarioBros. cover looks compared to the actual in-game sprite.
Is this a beta sprite?
Also, I noticed that it uses colors straight from the Arcade MarioBros, like the werid orange hair is the boots colors, it shares the same skin and brown colors too.
And this Beta sprite also has more palette range than the in-game one, just like it happened to the Arcade versions of MarioBros. and DK compared to the NES versions.
So, was SuperMarioBros originally supposed to be a arcade game, and the cover sprite is the scrapped Arcade sprite?