I have been looking at buying a Leupold with the CDS dial system. The website states that there only good for about 2000 in elevation and 20 degrees + or -.

Well I might shoot a hog in the summer time at 80 degrees or one in Jan. at below freezing.

So I have been doing some reading and at first glance it appears. At hunting distances it just dosent change that much. I was just playing around with a ballistics calculator and the difference in 60 degrees and 90 degrees at 300 hundred yards is .4".
Am I missing something?

Logic and reality have nothing to do with advertising lies

You are right about the slight difference in air temp.
Where you would see an obvious change is in elevation.
In altitudes above 2,000 feet the bullet trajectory is effected to a much greater deal due to the thinner air and in some areas much less dense atmosphere.
I live at 7,000 feet and if you took your rifle and loads to a hunt here you would see that the bullet would be very high and right of your zero from a much lower elevation.
(right hand rifling twist)

Air temperature, humidity, and air pressure (altitude) all make a difference. But it's not worth worrying about with the typical hunting rifle at typical ranges.

According to the online JBM ballistics calculator, a 180 grain Sierra flat base bullet at 3,000 fps will drop 11 inches at 300 yards when fired at sea level under standard conditions. The same bullet fired at 8,000 feet altitude will drop 10.3 inches at 300 yards. Does an elk or mule deer really care about the point of impact difference being less than the width of your thumb?

Besides, as you increase altitude you usually get colder (and drier), which makes air denser and offsets some of the difference.

Now, if you go to 12,000 feet on a 100 degree day when the humidity is 98% and shoot at chipmunks at 600 yards -- well, you'll need to adjust your zero. Of course, under such conditions you'll also probably need to adjust your heart medicine.