A clear-cut number one is someone who beyond a shadow of a doubt proves that he's the top dog, good example of this is Connor McDavid. He played two years up at the 2013 WU18C's and was just amazing. At no point after that was it in doubt that he isn't the clear-cut number one for 2015 NHL draft. Media might have tried to bring Eichel to the equation, but it was just obvious that McDavid was the superior prospect.
Matthews might have won the scoring title at the WU18C's, but 9 of his points came in 10-0 and 13-1 blow outs against Slovakia and Germany. In the remaining 5 games he had 6 points, which isn't anything to brag about considering the fact that he was surrounded by very good players and played against his own age group. Furthermore, the level of competition wasn't particularly great, for example Sweden and Canada had below average teams by their standards.
Matthews wasn't better than Puljujärvi at the World Juniors, in other words he certainly wasn't in a class of his own there. Matthews might have been slightly better than Laine and Puljujärvi at the WU18C's, but it was his 2nd tourney at that level, so he had the advantage of being both older and more experienced. Now that the new season is starting up it's obvious that Laine and Puljujärvi have caught up to Matthews and might have even overtaken him.