For the 172 ......
The Supplemental Structural Inspection Program is valid for Model 172 airplanes with less than 30,000 flight hours. Beyond this, continued airworthiness of the airplane can no longer be assured. Retirement of this airframe is recommended when 30,000 flight hours has been accumulated.
I guess they looked at the fleet age and had to decide on a point to which they would do the calculations based on the average for model. That would explain why the 206 has a higher life, higher average TTIS compared with other models ....