Uniques are not all of the same rarity
When an item is generated rarity is determined first, it can be white, magic, rare or 1 of some number of unique rarity bands. exactly how many tiers of unique there are has not been publicly revealed.
If the item is unique, a unique is chosen from the chosen unique tier.If the item is not unique, then base type etc is determined.
In the case of chancing, base type never needs to be determined. however if a unique tier is chosen for which no unique of that base exists, then the item defaults down to a rare. So the existence of multiple uniques for a base only affects the probabilities of getting those uniques if more than 1 is on the same tier list.
So to be clear, in a world where infernal mantle did not exist, the chances of getting shavs would remain the same. All that would happen is you would get a rare on those rolls where you would have gotten infernal mantle.

Nov
I love when more people come to Path of Exile
Going spellcarser: firestorm witch, crit flame totem inquisitor, flameblast elementalist, essenge drain/contagion, blade vortex and so on.

Apr
PoE 3.14 Ultimatum Top Ranger Starter Builds
Poe4orb has also prepared the latest PoE 3.14 Ultimatum Top Ranger Starter Builds collection. Check out these most popular ultimatum builds of Deadeye, Pathfinder and Raider now!

Oct
Chaos Damage doesn't bypass Energy Shield
The way I see it, unique items are bad design. They put build-enablers into a terrible binary: not enabled, or best-in-slot. There's no room for gear progression in that gear slot under such itemization.

Jan
I'm not fond of how you NEED the Anima Stone jewel
With Flame Golem enchant you get 88 damage(58 without) and can add 3 of these effects on top: 26.1 cast & attack speed, 87 crit and 11.6 phys dmg reduction or 304.5 flat life regen.

Nov
Eternal Orbs will literally rarer than mirrors when the Awakening hits
but since these good rares will replace the role of the Eternal Orb, they will at least cost as much as what an Eternal Orb used to cost.