Same as with the Drillborer Disk:
The +block effect is overwriting the base block instead of adding to it.
I’d guess that this is the case for all shields which have +block, so here is a list of shields (I didn’t look them all up on your page, but I’m making an educated guess) which have such an effect:
Drillborer Disk (fixed)
Barrier Shield
The Face of Death
Aegis of the Blood God
Elementium Reinforced Bulwark
The Immovable Object
Blessed Qiraji Bulwark
Lord Blackwood’s Buckler
Intricately Runed Shield
Petrified Bark Shield
Earthen Guard
Avalanchion’s Stony Hide
And maybe: Random enchant shields