A cadre of lawyers spinning out near-identical cases accusing employers of shortchanging pensioners has made it their mission to drag benefits calculations into the 21st century.

But many plan administrators remain wedded to the actuarial data they’ve relied on for decades—and they have the government’s blessing to continue doing so.

At issue in the four legal challenges (and counting) advanced by attorneys at Connecticut-based Izard, Kindall & Raabe LLP and Bailey & Glasser LLP in Washington is the...