A consensus standard happens by multiple stakeholders sitting down and agreeing on what everyone will do the same way. And agreeing one what they won't all do the same way. The things they agree to doing differently don't become part of the standard.
With compilers, different companies usually do things differently. That was the case with C87. The things they talked about but could not or would not agree to do the same way are listed as undefined behaviors. The things everyone agreed to do the same way are the standard.
The consensus process reflects stakeholder interests. Stakeholders can afford to rewrite some parts of their compilers to comply with the standards and cannot afford to rewrite other parts to comply with the standards because their customers rely on the existing implementation and/or because of core design decisions.
With compilers, different companies usually do things differently. That was the case with C87. The things they talked about but could not or would not agree to do the same way are listed as undefined behaviors. The things everyone agreed to do the same way are the standard.
The consensus process reflects stakeholder interests. Stakeholders can afford to rewrite some parts of their compilers to comply with the standards and cannot afford to rewrite other parts to comply with the standards because their customers rely on the existing implementation and/or because of core design decisions.