Steep Grade Ahead??

On Intestate 15 through the Virgin River Gorge, southbound, near the south exit of the gorge is a flat and level section of the freeway. This section of the freeway is in Arizona (the Arizona Strip). There are two yellow caution signs warning truckers of an upcoming downgrade, but there is no downgrade. The road is flat and level with no downgrade. These signs have been there for years, even replaced at least once, to my knowledge. Every time I drive this part of I-15 I try to figure why the signs are there and what logic keeps the signs there, and what justifies the spending of tax dollars on warning signs which are completely false.