I did some tests this afternoon and there are two things of which I'm now convinced.
A. Never mind this one. I forgot that since Shield slam is affected by armor, most likely the difference here is caused by armor value of target dummies.
B. SS damage does not scale 1:1 with Block Value. It's closer to 2:3 (For every point of 3 points of BV, you get 2 points of SS damage)
My first test was removing all my armor except
[Shield of Impenetrable Darkness] and equipping a 1h gray mace (in case 1h wep spec only took effect while a weapon). With this I had (per character sheet)
Strength -178 (adds 77 BV)
Shield BV - 122+21 = 143 BV
BV per tooltip of 287 [which equals (77+143)*1.3]
Shield Slam tool tips says "causes 603 to 635 damage modified by block value" note: This is exactly 10% higher than wowhead tool tip (549 to 577) which makes me think 1h Weapon Specialization is accounted for in tooltip.
After a LOT of trials (over 100 shield slams) I came up with an
average Shield Slam of 565.1 (note: if you do this test yourself, don't use devastate to proc SnB because the sunder effect ups the SS avg damage a lot)
Then I put on some gear and had
Strength 374
BV per tooltip of 769 [
interesting note, the 5% BV from meta gem is additive to the 1.3. To get right numbers the formula was base BV * 1.35]
Shield Slam tool tip still says 603 to 635 damage
After numerous trials here (not quite a 100 though) I got an
average Shield Slam of 888
Trying to put this into the conventional formula didn't yield any results so I recalculated what the actual base damage done was using these formulas (SS = Shield Slam Avg, BD = Base Damage, BV = Block Value, z = modifier, 2nd trial numbers have a 2 after them)
SS = BD + (BV * z)
SS2 = BD + (BV2 * z)
BD = SS - (BV * z) = SS2 - (BV2 * z)
Since the two equations are equal, we can solve for z
z = (SS2 - SS) / (BV2 - BV) = (888-565) / (769-287) = .67
Taking z = .67 we can solve for original equation
BD = 888 - (769 * .67) = 372.77
BD = 565 - (287 * .67) = 372.71
I did a couple of other tests with different block values (and less samples) and I consistently got BD = 373.
What this may not account for accurately is the z modifier could include the 1h wep spec or other multipliers. But bottom line, the value used is .67 even if it includes other modifiers.