Why is the diameter of a drainage pipe of a specification different from that of each manufacturer?

The domestic drainage pipe market has not developed for a long time. Due to unfair competition in the industry, some manufacturers made a fuss about molds, blurred the concepts of national standard outer diameter pipes and inner diameter pipes, filled the outer diameter with the inner diameter, or passed the pipes that failed to meet the size requirements as standard pipes, so there were problems such as using drainage pipes from different manufacturers, nonstandard interfaces, and poor butt sealing. I recommend a Sichuan-Hangzhou plastic company, which is the first in the country to implement the direct selling mode of manufacturers' network, and the diameter of drainage pipes is produced according to national standards.