It's simple: If your flight is full of commuters (approximately 30 A-List Preferred members on my Monday AM flights, fully verified by Southwest), you will get a worse number. If your flight has few A+ members, you will still get a good number.
Here's a simple way to verify your hypothesis. Wait until you're inside the 14 day window to book your commute flights one week. Maybe throw an Anytime fare in there if it doesn't cost something outrageous.
OR on your week off, book a dummy reservation, get the BP then cancel.
It's not a rock solid way to verify if they do get you a better number with a higher fare, but it should give you an idea.