Full Idea
In the possible worlds account of conditionals A⊃B is not sufficient for A→B. If A is false then A⊃B is true, but here nothing is implied about whether the world most like the actual world except that A is true is or is not a B-world.
Clarification
⊃ is material implication, equivalent to ¬AvB
Gist of Idea
Possible worlds account, unlike A⊃B, says nothing about when A is false
Source
Frank Jackson (Conditionals [2006], 'Possible')
Book Reference
'Blackwell Guide to Philosophy of Language', ed/tr. Devitt,M/Hanley,R [Blackwell 2006], p.215
A Reaction
The possible worlds account seems to be built on Ramsey's idea of just holding A true and seeing what you get. Being committed to B being automatically true if A is false seems highly counterintuitive.