The optimal skill level requirements for storehouses are higher than the requirements for the exact same materials for a silo. For example:
T2 storehouse - Iron sheeting optimal 425
T1 silo - Iron sheeting optimal 400
T3 storehouse - Steel sheeting optimal 650
T3 silo - Steel sheeting optimal 600
T4 storehouse - Cobalt sheeting optimal 875
T4 silo - Cobalt sheeting optimal 800
The T1 storehouse has the correct optimal requirements. I didn't check the T5 or T6 but assume these will also have wrong optimals.