>Rather than trying to specify the exact algorithm for everything (which is what the JS spec typically does, for example), they chose to describe the Zend model (or close enough) and say “it has to appear to work like this”.
To be fair, the JS spec may say those things but no modern JS engine actually does it that way. Despite the wording, "must appear to..." is the common interpretation of that spec. It is nice that this one makes that explicit, though.
On the other hand, there's been a fair bit of work (esp. for ES6, and to a lesser extent ES5) to minimize the number of things the specification states contrary to implementations.
To be fair, the JS spec may say those things but no modern JS engine actually does it that way. Despite the wording, "must appear to..." is the common interpretation of that spec. It is nice that this one makes that explicit, though.