We know Adam went to Aglionby Academy after he thought he saw a guy that looked like Ronan in a grocery store pay for all of his items without even looking at the price. Adam later thinks it silly to believe it was Ronan; but what if it was? What if Ronan swiped his card without looking at the price at checkout because he was looking at this cute boy that had just walked in?
Soon that boy starts going to Aglionby, but Ronan doesn’t think much of it. Now however, he knows he created Matthew when he was only a little kid. He’s starting to understand his power after the event of the second book. So now, in Cabeswater, Adam proclaims he knows when he’s asleep and when he’s awake. But Ronan start to worry; “Did I dream up this beautiful boy after basing him on someone I saw in a grocery store for five seconds? And he just happens to start going to our school?”
So when Ronan says to him, “maybe I dreamt you”, he’s not saying it like a sarcastic a-hole, but with a bit of fear.
Basically, these two star cross lovers met once without ever truly being sure, and based their own beliefs based on this one moment. Nerds!