No Guard Thank Guard!

[Reika]

Deflection and the other shield abilities put your "Guard" up, which gives you 100% chance to block - full or partial.

Incorrect, you can still fail to guard with guard, deflection, aegis boon, or Outmanuver.
Just use your battle log. It will read 'Your Block Fails' or what not、　And you will take full damage when that happens regardless of what skill is active.

and Note that Scutums have 99% block rate,　kite shields have 90%, so if youre a scutum user you may have never noticed the difference, only 1% chance not to block.