Martin,
Surprised at YOU asking that . This is because different manufacturers have different sizes for the same tyre "size" - they round them up or down. For example a 235 tyre might actually be 240mm wide or a 225 might be 220mm wide. Depends on the tyre type, the size given by the manufacture is just a "guide" and not definitive . And because the sidewall height is an percentage of the width, you can see then why they actually vary in daimeter and therefore circumference .

Also, it CLEARY states in the Stack instructions to measure the circumference NOT to calculate it (and that is because of the reason stated).