After my work with CGI (Cognitively Guided Instruction), SD Counts Mathematics, and Math Recovery Intervention Specialist, Math Recovery Leader training I have found that many of these domains of mathematics are not fostered to the highest extent that they could be.
I asked my niece once what 32-27 was and her first comment was "I can't do mental math." I told here to just try her best. Then she looked at the wall above my head as she crossed out numbers and borrowed. Then she finally got to the answered "5." I asked her to explain how she figured it out and she started writing number with her finder on the table how she crossed out and borrowed numbers. Then I asked her if there is a different way to figure it out. She replied "no" with a questionable brow, looking at me as though I was crazy to think there was another way.
Unfortunately, she has not mastered structuring numbers, decimalizing, or unitizing.