Coming from a successful rust-preventing user of both WD-40 and Ballistol in the decidedly humid Louisiana, my theory is that:
A) The aerosol version of both WD-40 and Ballistol are less effective at preventing rust than their non-aerosol equivalents.
And/Or B) Some people just have rust forming acidic sweat and must take extra care.
Years ago, I had a brand new S&W Model 29. I had been babying this thing since, not only was it my first new handgun, but my first new gun period. I'd pull it out almost every night, gaze at it, wipe it down, put it away. My brother came over and I let him hold it. I put it away but did not wipe it down first- don't remember why. Not more than a day later I pulled it out and found surface rust all down the backstrap.
I'd never had that happen before nor have I since. The only thing I can figure is that my brother's skin / sweat is more acidic than mine and caused really quick rusting?