Why is there space under 12 x 12" gable vent?

Hole is 12" square. But the galvanized vent has a slope 9n the bottom which would leave an inch wide gap when the vent is installed. This would be hard to imagine by someone not familiar with the vent (from Home Depot). I rememied the situation b cutting a piece of redwood to fit, but do not understand why... show more Hole is 12" square. But the galvanized vent has a slope 9n the bottom which would leave an inch wide gap when the vent is installed. This would be hard to imagine by someone not familiar with the vent (from Home Depot).
I rememied the situation b cutting a piece of redwood to fit, but do not understand why the vent is built as it is.
A gap that site, with no screen, would be vulnerable to bugs, rodents and last but not least, fireballs. (Wild fire area.)
0