Same Game Multi bets are calculated differently to standard mutli bets because they take into account the correlation of each leg.
A normal multi bet allows you to select a series of sports and/or racing legs and combine them to increase your odds and potential return.
Example:
You want to place a $10 bet (stake) on the 3 legs across Racing, AFL & NBA.
Leg 1: Racing - Nature Strip to Win at $1.50
Leg 2: AFL - Carlton Blues to Win by 10-20 at $2.50
Leg 3: NBA - Boston Celtics to Win at $1.90
Multi bet odds are calculated by multiplying the odds of each selection/leg. Normal Multi bet odds = $1.50 x $2.50 x $1.90 = $7.13.
Multi bet returns are calculated by multiplying the amount bet (stake) by the calculated Multi bet odds. (Stake x Multi bet odds = $10 x $7.13 = $71.30).
In the above example, Nature Strip winning their race has no impact on Carlton Blues to win by 10-20, however with a SGM, the legs involved are related and can have a direct impact on each other.
A standard Same Game Multi bet allows you to multi up a number of markets within a game to increase your odds and potential return. However, the reason the odds not look correct on face value is due to how each leg interacts with each other.
Example:
You place a $10 bet (Stake) on an AFL 3 leg Same Game Multi at $4.00 odds.
Carlton to Win at $1.10
Charlie Curnow to Kick 2+ Goals at $1.30
Patrick Cripps to have 30+ Touches at $3.45
Because each of these legs impact the likelihood of them occurring, the odds are calculated differently to a regular multi.
If Charlie Curnow manages to kick his 2+ goals and Patrick Cripps has 30+ touches, it's is much more likely that Carlton will win the match as well.