There has historically been no great consistency in the calculation of mitigation factors. Some blocking factors have been additive, some multiplicative. ZOS may have cleaned this up and they're all multiplicative now. I thought I saw that in some patch notes a while ago. When I last tested however, albeit a few years ago by now, it worked as follows (to the best of my recollection):
Factor 1:
Base mitigation (50%)
Factor 2:
Class skills (10% DK passive, up to 20% Arcanist skill, 40% sorc skill)
+ 1H/Shield / ice staff passive (20%)
+ Heavy armor (up to 7% passive)
+ Footman set (8%)
+ Defensive Stance (10% passive)
Factor 3:
Bracing Anchor CP (20%)
+ Passive CP (4% - I forget the name)
Total block mitigation = multiply factors 1, 2 and 3 together. Example:
Factor 1 = 1 - 50% = 0.5
Factor 2 (example) = 1 - (10% DK + 20% 1H/Shield + 7% all heavy armor = 37%) = 0.63
Factor 3 = 1 - (CP 20% + 4%) = 0.76
Total in this example = 0.5 * 0.63 * 0.76 = 0.2394 = approx 76% block mitigation
Regardless of whether it still works as above or whether all factors are now multiplicative, it's likely by pure chance that Bracing Anchor came out at exactly 10% in your build, or appeared to come out that way. The Advanced Stats page hasn't always been accurate. For example, if you slot Defensive Stance, the additional block mitigation hasn't been reflected on that page in the past.