Everyone loves topping tournaments.
Yes, its not everything in the game, but saying its not something everyone enjoys is wrong.
So you pick up your Lightsworn/Anti-Meta/Blackwings/Zombie/Gladial Beast deck, and head to your local friendly tournament store, and play, play, and play.
But you lose.
You have perfect build, tested over and over, with super high chance of OTKs and next to no chance of dead draws.
Your team has crunched numbers and figured out every single percentage of drawing comboes and other cards.
On top of that, you didnt misplay a single time. You are considered by many to be one of the best players out there in terms of plays and thinking, and deck building.
But you still lost.
The answer is luck.
In deck building and playtesting, what you analyze is probability, and how to maximize it, furnishing your deck to have the highest probability of drawing card X or combo Y.
But when it comes down to the tournament, and your games that matter, high probability and good decks really help, but the reason people win or lose is ultimately luck.
Theoretically, you could calculate the probability of you winning with the entire decklist of your deck and the endless comboes and the decklist of the opponent,
but that is just theory. Even if you have a 99.9% of winning, luck can still take over and give the opponent that 0.1%.
That being said it wont happen very often and probally not in tournaments due to other factors(Misplays).
Thus, when building decks, be sure to think of the probability, but do not forget that when it comes time to shine, everything is the luck of the draw and the roll of the dice.