SQLite odd behavior with NOT LIKE

Using the Skype SQLite database, and DB Browser for SQLite (v3.7.0)

This returns 13 records:

select * from messages where chatname not like '%pdx.%';

This returns a count of 0:

select count(*) from messages where chatname not like '%pdx.%';

This doesn't delete anything:

delete from messages where chatname not like '%pdx.%';

Does anyone know why SQLite might do this?

I've tried with several different SQLite front-end tools, and all seem to do the same.