Skill Bonuses on equipment can be +3, on Epic- and (assumably) Unique-rarity gear.
But the way things work on DKS is this: Each "unique" item (and by that, I mean items with gold names and no rarity listing, NOT Unique-rarity items) still has a rarity value, it's just hidden. Most unique stuff has a skill bonus in addition to their normal rarity.
For example, the Adventurer's Band ring in Broken Valley Village (sold by Lamotte) has +2 to Critical Chance and +1 to Regenerate skill. The Regenerate bonus is the true "extra" on the ring. Otherwise, it is basically a Rare-rarity ring: one (small) stat bonus. Because rings have five different bonuses, it is one of them (crit chance, HP/Mana regen, and direct HP/Mana bonuses are all to be found on any ring).
Because Pendants fall under Jewelry w/ skill bonuses (like earrings and bracelets), the Talisman of the North has its special skill bonus (Wisdom +2), which is unchanged from ED. Then, it has two additional effects, a normal stat bonus, and a normal skill bonus, like other amulets. But since the item is entirely pre-determined (its stats aren't randomized), they put the extra +1 to Wisdom in the "skill bonus" slot on the item. So Raze is partially correct in that it's likely a programming issue, but not quite correct in the specifics, at least from my observations.
Note: Interestingly, many unique amulets/bracelets/earrings have a stat bonus and a +1 skill bonus, when normally this is impossible: rare and magical items, the only ones that have +1 bonuses, never have another stat on them, and heroic, which DO have a skill bonus and a stat bonus, only have +2 bonuses. These are the only items with such discrepancies, though, and all other unique items follow the rules properly for the most part, except one Warhammer which has Sword/Axe bonuses (Melee stats, instead of Magic, like Warhammers usually have).
