Laying all odds could be sure profit, like backing all odds is not always sure profit.

I assume you are saying this because of the commissions involved?

If by commissions you mean the bookie's advantage then yes. The bookie's advantage comes from not offering odds which represent the true chance of the outcome occurring.

Eg, considering a football match (the event) that can be either a 'home win', 'draw' or 'away win' (the outcomes) then the following odds might be encountered to represent the

*true* chance of each of the three outcomes:

Home: 1-1

Draw: 2-1

Away: 5-1

When you convert those odds to probabilities you get

Evens (or 1-1) corresponds to a relative probability of 1⁄2 (50%)

2-1 corresponds to a relative probability of 1⁄3 (33 1⁄3%)

5-1 corresponds to a relative probability of 1⁄6 (16 2⁄3%)

By adding the probabilities together a total "book" of 100% is achieved, which represents a "fair" book. The bookie, in order to give himself a profit, will invariably reduce these odds, so making the bets unfair to the punter.

e.g, he might set the odds to the following:

Home: 4-6

Draw: 6-4

Away: 4-1

4-6 corresponds to a relative probability of 3⁄5 (60%)

6-4 corresponds to a relative probability of 2⁄5 (40%)

4-1 corresponds to a relative probability of 1⁄5 (20%)

By adding these percentages together a book of 120% is achieved. The 20% increase over the fair odds represents the bookie's "house advantage".

This is no different from roulette. You might as well say that because some dozen system isn't winning then you should switch to betting on the other dozens + the zero, then you're sure to win! Nope, because the house edge exists for every possible outcome.

However with sports betting it's easier to find value bets than in roulette because neither the probabilities or the odds (payouts) are fixed. The bookies can't always "balance" their books so that all outcomes show a profit for them equally, and therein lies the opportunity for the sharp sports bettor.