Good question. I agree with your math, based on the assumptions provided by Consumer Reports. However, I do not agree with their assumptions. It looks like they were lazy and took the average lifespan of the front and rear tires in creating the lifespan for a rotated tire.
To illustrate, consider a driver who starts with four new tires. He then switches the front and back tires after 15,000 miles. Before switching, the front tires would have worn down 75% and the back tires 25%. After switching, the front tires will have 75% of tread left and the 25% on the rear. He then drives another 15,000 miles. Again, the front tires will wear down 75% relative to a new tire and the rear 25%. That will leave both with zero tread at the same after after 30,000 miles. Not 40,000, as Consumer Reports claims.
To get the correct lifespan by rotating, one should take the average of the tread wear. In this case we average 1/20,000 and 1/60,000 to get 1/30,000. Then take the inverse of that average wear per mile to get the longevity of rotated tires, which is 30,000. Or you can use the formula 2*F*R/(F+R), where F = lifespan of front tire and R = lifespan of rear tire.
This question is asked and discussed in my forum at Wizard of Vegas.