While I doubt this actually explains the (admittedly funny) error, I will say this: you do notice it. And, you do think about it. Which is more than you can say for most warning signs.
Back in Florida, there was a park near my house that had a “Speed Limit 9 MPH" sign. Every time we saw it, we’d laugh at how dumb they were not to post a ““normal”” speed limit that ends in a 5 or a 0.
But, then, we realized that, yeah, we did actually see it. And, yeah, we did actually slow down to 9 MPH.
Put differently, how many of the hundreds of other warning signs you saw in the month of July can you remember this clearly?
Nope. Me neither.