Well first of all "off the top of my head" is not a phrasal verb; rather, it would be a locution. At first glance I would say that it purports to be an emphatic way of saying " I can absolutely say that ... " I can definitely affirm that... ".
This is just the way I perceive that proposition. But I am not entirely sure.
I never used it before. it seems really colourful.
Student or Learner