Non-bundle games being wrongly recognised?

Nope, nothing like that.
https://www.steamgifts.com/bundle-games/search?q=marble+mayhem

It is simply on the bundle list itself, not through an error.

I'm assuming it has to do with the lowest price, it likely hit the required 95% discount in some region for addition to the bundle list.

bundled games on sg don't have the same definition as bundles on steam or 3rd party sites.

https://www.steamgifts.com/about/faq

Giveaways for bundle games created after the bundle date will always receive 15% of their value. For example, if you gift 2 copies of a $20 bundle game, you would receive $6 (3 + 3) in value towards your level. Keep in mind that changes to the list of bundle games will apply retroactively, so if a bundle begins today, attempting to gift the game before it's added to the list will offer no advantage. Games may also be added to the bundle list if they're available for free during a promotion, or if they're highly discounted in a sale.

this includes discounted games in all regions (RU game is -95% in US store, it gets bundled), prices glitches, stackable coupons.

Thanks for the clarification :)