Huh? The example is for PostgreSQL's "serializable" isolation level, and that's what I'm confused about.

Serializability should ensure that the outcome is equivalent to some serial execution. What serial execution of those five transactions yields [1], [1, 10], [1, 1, 10]?

But also, I just read that prior to PostgreSQL 9.1 (released in 2011), the "serializable" isolation level was actually just snapshot isolation (now called "repeatable read"). So maybe that's what benmmurphy is referring to?