Physical resistance, or PR for short, is one of the most hated abilities when encountered on a NPC and one of the most critical ones for a tank as it reduces directly the percent of physical damage taken.
Calculating the different factories together is not simply adding them all up. Player characters can't achieve 100% PR while NPC's might do. If wearing one item that gives 8% and another that gives 5%, it sum's up for (0,92 * 0,95 = 0.874) 12,6 %, not to 13%. Also PR from boons, race and items sums up differently.