Your assumption is quite logical.
I don't have the book itself in front of me right now but if it is a lookup table, either you fill it using INSERT INTO commands, or you import the data from a CSV file or other type of text file. Either way, at some point, you had to insert the data manually.
Other solution could be to extract these values from some other table using a SELECT query with a DISTINCT attribute so you will not get repeated values in the result. That is, of course, if the values where already in other table