This leads me to a few different possible conclusions:
A) The sign means, "Hey you, 20 mph if you don't see a sign. You know, like *this* one right here that says 45 mph. Yeah, that means you can go 45, but if that wasn't there? You'd only be able to go *20*. Got it?"
B) The sign is a punishment for the rare 'half-perceptive' person. The ones that see the 20 mph unless otherwise marked sign, but then completely miss the 45 mph, so they wind up going the 20. Of course, this is also a punishment for all those cars behind them. *Pauses a beat* Actually, that would explain so much.
C) The person ordering these 20 mph signs mistook the "Bob, make 50 of these signs for Boise and surrounding areas" note by adding a 0 to the end of the 50. Not knowing what to do with all these extra signs, they decided to place them wherever they could so their tax money wasn't lowered the following year for having excess supplies.