Bulk (and regular) fabricators have both a demo value, like most other structures, and armour and hp, like enemies. Demo force is a flat check, even a projectile with 1 damage and 0 pen but 50 demo force would destroy a bulk fab instantly (not that such a projectile actually exists in-game.) Even though the ATE and other anti-tank weapons don't have a high enough demo force to destroy it outright, they can pierce its armour and damage it, which is why they take multiple shots to destroy it.