Exactly that
decks never rust out from the underside
They rust through from the top
and should be blown clean using an air duster
On ventilated decks the water from the deck wash ends up on the top of the deck where it sits and rust out the bolts and the deck itself
On the underside it ends up in the bearings emulsifing the grease & destroying the bearings
Bacteria in the grass packed in under the deck consume all of the available oxygen then start anerobic decomposition
No oxygen = no rust
The grass on the top of the deck is loose packed so adsorbs water, & holds it there for the deck to rust & electrolysis to corrode the spindle mounting bolts.