No, I'm not talking about sales.
I have noticed a trend of games like Starbound, Stardew Valley, Enter the Gungeon with cheaper prices when compared to Steam. Any reason for that? Not complaining, just curious.

Yes. Because Steam usually doesn't apply regional pricing before local currency is supported, while GOG does in the case of Mexico and South American countries. Thus, in your case, Steam sells you the games with the US price, while GOG groups you with Brazil/Mexico and sells you the games with the lowered price.