Bookmakers do not offer fair odds. Each price is shaded slightly below its fair value so that, whatever the result, the book keeps a percentage. Sum the implied probabilities of every outcome and the excess over 100% is the margin — also called the overround, juice or vig. Typical football match-result margins run from about 2% at sharp bookmakers to 7%+ at recreational ones.
Margin is the default headwind every bettor faces: betting randomly, you lose the margin over time. That has two practical consequences. First, comparing prices across bookmakers matters enormously — the best available price often erases most of the margin. Second, beating the market means overcoming the margin and then some, which is why genuine long-term edges are rare and small.
Two-outcome market priced 1.91 / 1.91: implied 52.4% + 52.4% = 104.8%, a 4.8% margin. The fair price for a genuine 50/50 would be 2.00 / 2.00.