...so why is it that a Christian group paying for pro-religion billboards feels it necessary to just make shit up?
The blog Computing Intelligence has a suitable riposte:

"There is nothing so simultaneously dull and mentally detrimental as spending your Sunday morning in a church." - George Washington, 1st President of the United States of America.
Haven't heard that quotation before? That's because I just made it up. Chances are, George Washington never said that, but a lack of documented evidence for the attribution of a quotation is apparently no problem for some people. It would seem that a pair of theocratically minded citizens of the United States decided it was perfectly reasonable to make up a sentence that corresponded to their beliefs, and then slap that statement on a billboard and attribute it to George Washington.

However, if this is the way that some Christians want to behave in furtherance of their ideas and beliefs, I look forward to either calling BS on this, or getting into the business (like Computing Intelligence) of Making Stuff Up. Given the razor-sharp wits available at websites like Pharyngula, I am sure that we can out-BS them.
Bottom line though: these billboard charlatans are deceitful sacks of shit, hypocritical lying loons.
UPDATE - No sooner have I written this than another example pops up, this time from Oklahoma...you cannot make this stuff up.