Quoting from the first photo example:

"In the early 17th century, buildings were taxed by the number of floors leading up to the cornice of the roof.

Architect François Mansart ingeniously responded to this rule by hiding an additional story or two under the roof for additional living space, rendering it exempt from taxation."

...and hence the "distinctive architecture of Paris, with its mansard roofs".

You have to read the article, not just look at the pictures.