There were a couple patches for the game, so if it were a mistake you'd think it would have been fixed. Also, if it were a misplaced decimal, the values for the rest of bonuses would be off (0.5, 0.6, 0.75, etc make sense, but the incremental increases for 0.5, 0.53, 0.56, 0.59, 0.62, etc seem a little small).
Even without that belt, any character can effectively have an unlimited mana pool (though I have not tried a mage build).
Playing a ranger in DKS, I found a few levels of Mana Efficiency (level 6 at the end of the game) and Increased Mana enchantments (once I started getting armour with 2 enchantment slots) pretty much eliminated mana restrictions, even without boosting Intelligence or Spirit.
In the original release of D2:ED playing a warrior, I never bothered with high mana skills like Thousand Strikes, and level 1 of Mana Leech was more than enough to keep me from running out. I learned level 5 of Mana Efficiency so I could summon my creature and have enough mana left over to cast the Healing spell soon after if I needed to.
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