The 60-grain Ballistic Tip was designed to stabilize reasonably well in a 1-12 twist--which is why it isn't much longer than the 55, and probably why its BC is only slightly better than the 55's.
Nosler, like many bullet makers, often suggests rifling twists faster than required for certain bullets, to make damn sure they stabilize. Which is probably why they lump the 60 BT into theier "60-62 grain. fast twist" data for the .223.
