Learn / The bookmaker's margin
The percentages always add to more than 100. The extra is the fee.
Take an evenly matched rugby league game. A fair coin-flip market would price both teams at 2.00 — each a 50% chance, summing to a tidy 100%. But look at a real market and you'll see something like 1.92 / 1.92.
Flip those into percentages (1 ÷ 1.92) and each side claims 52.1%. Together: 104.2%. Reality can't be 104.2% likely — the extra 4.2% is the bookmaker's margin, baked into the prices. Whichever side you take, you're paid slightly less than the true risk deserves.
At 1.92 on a genuine 50/50, a winning $100 bet returns $192 instead of the fair $200. Do that every week and the margin — not bad luck — is what drains the bankroll. It's the same reason covering every outcome of a market at one bookmaker guarantees a small loss: you're paying the fee on every leg.
The margin isn't fixed — it varies by bookmaker, sport and market, and it only fully applies if you take one bookmaker's prices as given. Different bookmakers disagree, and the best price for each outcome usually lives at a different one. Shopping around rebuilds the market at the best available prices, which can cut the effective margin by more than half — and on rare occasions push it below zero, where covering every outcome actually locks in a profit.
See it in numbers: put both sides of any real market into the cover every outcome calculator — the margin figure it reports is exactly this surplus.