[cromag]

The two examples you've chosen are series. In the first book of a series you have to come up with characters, settings, backstories, etc., plus the story. For the remaining books in the series you have an established base to build on.

For someone who writes non-series novels, the second, third, etc., books probably reflect an increasing skill level, so I imagine they'd still be a little faster for the writer to produce, but I couldn't prove it.