The book Practical English Usage by Michael Swan says that the sentence
When I had opened the windows, I sat down and had a cup of tea.
is more natural than
When I opened the windows, I sat down and had a cup of tea.
because the first action is separate from the second. But I cannot see how it is separate. The same is with
When I opened the window, the cat jumped out.
When I had opened the window, the cat jumped out.
In this pair of sentences, however, it says that the first one is more natural. And it also says that it should be
When I had answered my emails, I did some gardening.
not
When I answered my emails, I did some gardening.
Why? Help me please to see the difference.