You aren't really missing anything with this quote.  It is simple teasing conversation between the two.  Bella wakes up trying to remember a dream.  She then realizes that Edward is in her bedroom and she can't believe he stayed.  She jumps in his lap and says,

"I was sure it was a dream." 

"You're not that creative," he scoffed."

The ending of the sentence is what gives the reader the knowledge that he was joking with Bella.  "He scoffed," means he was joking, kidding or teasing Bella.