(assuming you get synced to lv20) Non-sync players often have gears not up to their current level, like having a lv17 gear for a lv20 class so those gears will not be synced as it didn't go beyond the dungeon lv cap, an endgame/higher level player will most likely have all theirs synced to 20 so they get the maximum VIT and other stats in each gear compare to non-synced and will often have more HP than the Tank.

Also, you don't get to use the gemstone accessories until you're lv28-29 so a synced non-tank will seem to have more HP than the tank because the tank themselves lost a significant portion of HP because most if not all accessories before the Sphene set (it gives VIT bonus) doesn't have VIT at all.

All these are based on my past experience, hopefully it clarifies anyway.