Strict vs Returning variants with CommonJS support

[febeling]

The variant returnExports.js says:

// If you want something that will work in other stricter CommonJS environments,
// or if you need to create a circular dependency, see commonJsStrict.js

But it doesn't really explain what the strictness is about. What is violating CommonJS in the stricter sense, and which still seems to work in node, in this example?