What are the differences between std::decay and pass-by-value?

The specification of std::decay in N4296 leaves the following note:

[ Note: This behavior is similar to the lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) conversions applied when an lvalue expression is used as an rvalue, but also strips cv-qualifiers from class types in order to more closely model by-value argument passing. — end note ]

It seems to me that ideally std::decay would model by-value argument passing exactly, but for some reason it's not defined that way.

I think it could be defined in terms of template argument deduction in which case the implementation could also be defined to leverage template argument deduction to exactly model by-value argument passing.

template <typename T>
struct decay {
  private:

  template <typename U>
  static U impl(U);

  public:

  using type = decltype(impl(std::declval<T>()));
};

Questions:

1. What are the differences between std::decay and by-value argument passing?
2. Is std::decay designed to not model by-value argument passing exactly?
3. Would the implementation above model it exactly?