Quote from bwolinsky:
Atticus' point is that if the strike is $5, and the stock's at $5, delta should be $1. Well, sorry folks, it doesn't work like that. The probability FTR moves $1 is unlikely, and if it was likely, delta would be higher relatively speaking.
For normal markets, delta will predict what the move in the option will be under normal market conditions. His point is not well taken because he's saying if that is the CFA's definition then delta would be $1 or so, but delta is only showing a typical option price movement for an incremental move in the underlying.
Again, there's hundreds of examples of this, but unless you watch the change in delta or realize delta's relatively stable those aren't what the predictions of delta are for.