Multivariate statistics

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/10/static/multivariate-statistics-examples.html
Description:

Hi All, 

I'm a little confused by the documentation here: 
https://www.postgresql.org/docs/10/static/multivariate-statistics-examples.html.


In particular, the selectivity of the second query doesn't seem to match up
with what is expected in the notes. I feel like I may be missing something?


(I am referring to the output of this explain command: EXPLAIN (ANALYZE,
TIMING OFF) SELECT * FROM t WHERE a = 1 AND b = 1;)