Calculation vs. Evaluation
Although they go hand-in-hand, it's been my experience that in a typical OTB game, often the result is decided by somebody underestimating or overestimating a certain line. Both sides saw the variation, but the resulting position was a little bit unclear and one side correctly determined it favoured themselves. If that makes sense...
Here's the kind of position I'm talking about. Is Bxg3 a good move?
I'll let you guys argue about it for a few hours before I post the answer!