No.
I think we can't know for sure unless someone from EA Tiburon tells us the formula for this and how accuracy ratings play into how the computer determines where a pass will go.
I think it's a combination of the two things you said but it's more about times on target, especially within the relatively narrow range of QB DAC values (which vary from like 60-90something as opposed to the full gamut of 15-90something if you throw in all other positions). That was my intention by my post.
For example, a QB with low DAC throws plenty of passes that are like 20 yards past the receiver but he does often enough throw accurate passes that it can't entirely be a measure of variation off from the target. The fact that some of them are perfect completions indicates there is an element of the ratings representing how many times the ball will be on target.
I was just pointing out that when you're talking extremely low DAC players (like an RB with a DAC of 15), I have never seen an on target deep pass from such players. It seems to be wild every time but those with maybe a 35 DAC seem to be a little less erratic. However, I haven't seen enough such passes nor have I explored this topic, to make any absolute statements about it. It seems like in this case of really low DAC's, it's about how much variation is on the throw, but who knows?
For when it matters (between QB's), it seems the DAC rating is predominately about how on target any given pass will be and then from there, there's an element of how much variation.
But like I said, this is just from casual observation while playing. I haven't tested it.