The reason for this architectural oddity is a simple one - it makes it easier to haul household supplies using a pulley system set in each house's gable above the attic rather than hauling them up four flights of narrow stairway.
The lean ensured that any cargo raised through the pulley outside the house would have less chance of smashing the houses!
Attics were once used to store valuable goods, spices, cotton and cocoa, where they will be dry and safe from the frequent floods.
Case in point (although not by any means a normal person's house, but it was one of the few they let us poke around): the Van Loon House. The Van Loons were a noble merchant family. Willem Van Loon was a founding member of the Dutch East India Company, and a grandson became mayor of Amsterdam.
The Van Loon's house occupies the entire width of the block, a space normally occupied by two houses. The family's main house faces one canal, the Keizersgracht, while the coach house faces another canal behind, thus affording their horses a better view of the canals than the normal Dutchman.
Sitting room that leads into the garden. One can see the entrance to the coach house.
Standing at the coach house, looking at the main house.
And the view opposite, looking at the coach house. A properly large garden in between.
Right about this time the big K decided that we had had enough of culture and history for one day. And besides, he was hungry. Again. So it's off to market!
Next post: Two Little Piggies Go to Market