Variance
How widely your real results swing around your true win rate.
Variance is why a winning poker player can lose for months. A winning NL100 player with 5 bb/100 win rate and 100 bb/100 standard deviation has a ~16% chance of being down after 10,000 hands. After 100,000 hands, still ~6%. That's the noise you're playing through.
For MTTs, variance is brutal: a player with 20% ROI can lose money over 40,000 tournaments. Most live grinders won't play that many in a decade.
The takeaway: stop asking "did I win?" after a session. Start asking "did I make good decisions?" The first question is dominated by noise. The second is the only one you can control.