[The Rest of Your Life] Listing 1 low noise

[genmeblog]

The image generated in listing 1 (or listing 3 in version 4.x) contains very low noise. My result however is quite noisy.

In book v.4, the only difference is samples_per_pixel set to 100 (instead of 64 from beginning of the book). It doesn't create such smooth image as here.
In book v.3, pdf can have negative values and the result is very noisy.

Book v4 version from my code:

Is it bug in the book or in my code?

(samples_per_pixel set to 1000 gives the result similar to this from the book)

[hollasch]

Converted to issue #1018