I do not know what you mean by stretch... Some tires flex under extreme centrifugal forces according to carcass design and engineered purpose...
Section width is NOT thread width... It's the widest the sidewall will go on mounted in the recommended width wheels. Tires are also inflated to maximum recommended pressure cold.
Also engineers account for the flat spot on the bottom of the tire due to capacity of load... Just because the side wall it's not 100% vertical does not mean the measurement in the design is less or it's wrong.
Having said all that, each manufactures has a different ruler to measure their tires. Coming from a Racing background were tires are as important as engine power, there is a lot of technology in today's street tires...