Well, it would start to make sense if you stepped back for a moment to consider the role Celebrini is playing on his team vs Hutson and the effect they have on play relative to the team around them.
Hutson is having a great year, no doubt, but is on a team that has performed above expectations across the board. He's able to have a leading role on the powerplay, while vets like Matheson, Guhle, and others soak up difficult defensive assignments.
Mack is a no doubt 1C at 18 (!) years old and has taken on a defensively responsible game while getting a higher PPG than any rookie in this class. Hutson is a fantastic 21 y/o defenseman, but he is very clearly still has a negative defensive impact on the whole. I get that he has takeaways are higher and such, but Hutson's O-zone starts are higher (65%) vs Mack (55%) and Mack has a better overall impact on his teams chances (51.3% CF) vs Hutson at 50.7% CF. Mack is controlling the puck more (on a worse possession team) and getting than Hutson is while getting less time in the offensive zone.
I don't generally rely on the advance stats to tell the whole story about a player, but in a strong Calder race like this one, I think deployment and impact make a difference. Eye test is one thing, and both fan bases will say their guy is the best, so why not look at objective measures?