In classic D&D, the hit die for thieves is a d4. That seems kind of harsh until you look at the XP progression. In the Skylands campaign, most of the party is second level. The mage and elf are still first. The thieves are third
So, here’s how it shapes up. The Skyland columns take into account that we’re doing max hp at first level.
So, if the thieves have hp totals rivaling the clerics. If they used d6 for hit dice, they’d be in fighter hp ranges.