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Is it true that bookmakers always win so there are surprises?

While they do not earn the same, the bookmakers are covered and pay the surprises with the money that the majority have wagered for the favorite. For example, if the No. 1 in the World plays with the No.200 of the World, most people will bet on the favorite.
If your opponent wins, the bet will not generate big losses. Moreover, if the stranger receives a number of unusual bets, the bookmaker will close the market and suspect a match (in tennis it has happened in several cases).

The bookmakers can afford to lose in some event, that often generates advertising and allows them to attract other customers. The other question is whether a certain bet can break a better bookmaker. It is very complicated to think about that, it has to be a big surprise, let’s think about surprising facts.
At the soccer level, the victory of Uruguay in Maracana 1950 … at that time there were no bookies, not even internet. Well … the odds of Uruguay could have been 10.00, 20..00 at the most. It would not have affected the bookmaker to the point of leaving it bankrupt.

The same for example with the day that Buster Douglas knocked out Tyson.
However, obviously if many people had bet that North Korea (1000 to 1) won the 2010 World Cup South Africa and that was … could happen.

In short, the bookmakers look for their profits, sometimes they have to lose but almost does not affect them.