Cowl Rust Cause?

[ursus]

I have always wondered why so many Model A's have rusted out in the lower cowl area on one or both sides of the body, usually both sides. Often, this may be the only part of the car that is rusted out. By rusted out I mean that the sheet metal needs replacement. What makes no sense is that there is no apparent pathway for water to enter or accumulate there.

Can anybody explain this phenomenon? My interest comes from wanting to prevent future rust in the repaired area. Thanks!