Each game has an associated hidden skill, and your Sim will get better as they play each of them. So if you spend all your time on one game in particular then your sim will become very good at that game and if entering competitions will have a greater chance to win.
There is a post here that explains the mechanics.
click This is based around the premise that your sim will be trying to enter competitions (Tournaments) to make money from the skill but the info is good regardless.
*Edited: if you don't play the game for some days then your sims hidden skills will decrease so if you're wanting to make money from this then you have to make your sims play their chosen game for a few hours every day.
