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Poor quality control at the factory. The car has screw adjusters to set ride height at each wheel, but they have to be adjusted correctly to give you the correct ride height. Obviously, a good number of the cars aren't correctly adjusted when they come off the line.
Don't know. You'd have to ask GM on that one. I started a thread months ago where a bunch of us measured our ride height by measuring from the ground to the top arch of the front wheel wells on level ground. I don't remember the exact measurements and the search doesn't work so I can't find the messages but we were all coming up with measurements within about a 1 inch to 1.25 inch range for stock cars. After looking at a few of the messages, we noticed a trend where it seemed the base cars had a lower ride height and the Z51 cars were actually higher. That may have been a bogus observation due to the low number of responses so take it with a grain of salt.
In any case, the measurements of ride height varied greatly within about a 1 inch range.
EDIT: I was able to find my old post in the archives. I think this was the post that had the measurements in it but unfortunately it looks like page 2/3 of this post have been "orphaned". Unfortunately in the process of archiving some of our threads, it looks like the forum moderators/techs have lost some old threads. This makes two of my posts in the last week or so. I had bookmarked a post quoting all of my 1/4 mile times too and that one has also been [it appears permanently] lost.