I suppose I should go a bit further and explain the difference.
In both cases, the first offense was an "impossible crime". Impossible crimes are still crimes of intent. For instance if you try to pick a pocket but there's nothing in the pocket. The fact that the pocket was empty made it impossible to steal anything, but it was still entirely possible for the pocket to not be empty, and for you to steal something. Therefore even though the crime in this instance was impossible, you still had the intent to commit a possible crime.
The second offense is an "inherently impossible crime", where even if you execute your attempt with all of your actions playing out exactly as you imagined it, it would still be impossible. For instance, Chris' plan that his dimensional merge will kill billions of people in fire. Chris is not guilty of attempted genocide, because his entire plan is inherently impossible even if he executes it perfectly.