Same data, different results in Postgres vs. FrontBase

I have a query that returns 569 rows in FrontBase, but only 30 rows in Postgres. The data is the same as I just finished copying my entire database over from FrontBase to Postgres.

I've reduced my problem to the following statement and have discovered that FrontBase returns null rows along with the rows that match the query and PostgreSQL only returns the not null rows.

Is that normal? I guess I have to rewrite my queries to handle this situation.

Does anyone have any idea why the two database engines might differ in this way?