My informed guess.
Weapons have a stat for weapon quality, when a block happens the weapon with the lower stat checks for breakage. If it passes the higher level one tests.
Any given type of weapon has a weapon quality range and default. A normal Broad sword might be weapon quality 12 and a normal club weapon quality 8.
Weapons that are enchanted might be un-breakable to normal weapons.
Shields have a high inherent weapon quality and relatively low cost and a high defensive bonus, so good for blocking.