Why the order of expressions in an if statement

Suppose I have a condition IF:

if (A || B) 
    ∧
    |
    |
   left
{ 
   // do something  
}

Now suppose that it is Amore likely to get the true value, and then B, why does it matter to me which one is on the left?

If I put them in brackets IF, then I know (as a code programmer) that both sides are necessary.

The fact is that my professor wrote on his lecture notes that I have to put a "more probable variable in order to get true" on the left.

Can someone explain the reason? ok i put it to the left ... what am I typing? lead time?