^^^ They might not have been able to get a convertible Vette at the same time. It's not as simple as it seems.
I've been through the same thing a hundred times in all the years I worked in the car magazine biz. Until you get to the MT, CD R&T level (in the old days, anyway) it can be difficult to schedule a comparison test. The more cars in the test, the tougher it is.
You're not just working to hit your deadline date, but also to fit in the car company's schedule for its test cars.
And, occasionally, you'll get the call (sometimes the day before the car was supposed to be ready) that the last magazine guy damaged it and it won't be ready.
So, as an editor, you then have to make the hard decision:
1. Go ahead with the coupe that they do have to loan you. So now it's a convertible vs coupe article.
2. Wait for an indeterminate time period to do the article the way you originally envisioned it (never a good idea, for a lot of publishing reasons.)
3. Just do a single car test.