Yu-Gi-Oh! Zexal/Fridge

Fridge Brilliance

 * The reason Yuma sucks so much at duels is because his family forbade him from dueling. He's in effect a complete beginner who has had to figure out the rules as he goes along.
 * Shouldn't make him completely unwilling to accept help though, and he's even worse than your basic duelist who had no training. Even they would have heard the rules.
 * Plus, for someone who was "forbidden to duel", Yuma demonstrated no difficulty in repeatedly dueling without consequence from his sister, the one who'd "forbade" him in the first place.

Fridge Logic

 * Yuma adds Numbers cards to his deck when he defeats a Numbers user. Numbers, being XYZ monsters, go in the "Extra Deck". The Extra Deck is limited to 15 cards. Yuma will have to pick and choose which ones go in.
 * It's not like he will summon more that 15 in a single duel though.