DK Orc Perfected Yolnakriin, Crimson Oath and Nunatak is the sets.

assuming you are running all gold, and sturdy on the shield, that would be -32% (4% per pc x 8 pc due to the shield) block cost from just sturdy trait

926 is your no-armor cost
733 is your cost with armor

so theoretically, you should be getting reduced 296 cost if it is calculating from 926 and not the base (assuming there is no other block cost reduction from cp, not sure of the order of the operations in where it calculates)

926 - 296 = 630

my guess is that this calculation is taking place before cp/skill passives, because i think the base is around 1100-1200 with no armor, passive skill, or CP reductions

im also assuming that none of your armor is medium armor either, as medium armor passive gives block cost reduction (heavy armor gives a bonus to block mitigation i believe)

i dont know how much i really trust some of the numbers in the advanced stats panel lol

as you noted some things do not seem to update those numbers correctly

i actually sometimes forget it even exists lol