warrior 2pc T4 bonus
"You have a chance each time you parry to gain Blade Turning, abosorbing 200 damage for 15 sec."
instead of the 200, i have been seeing 256.

warrior 2pc T5 bonus
"Each time you use your Shield Block ability, you gain 100 block value against a single attack in the next 6 sec."
normal blocks were going for 635, once i used shield block i was getting 765.

any clue as to why these set bonuses are getting extra values added to them?

That would make sence (30% of 100 = 30 so the 130 fits right in) however, it dont seem that the set bonus' block value added to shield slam dmg which is why i didnt think shield mastery would have any effect on it.

That is correct, Shield Mastery applies to the T5 2pc bonus. And it actually does increase your Shield Slam damage, but you have to be quite fast with SB>SS before the mob hits you.
No idea what's the deal with the T4 bonus.

The tier5 2set bonus does apply to your shieldslam damage.

You must also remember that 1handed spezialisation + Shield mastery and several other talents/buffs increase theese stats %wise, and not flat damage.

Therefor the 2set bonus is alot more powerfull then at first glimse