Your aptitudes are the natural ability to do something. It is about finding your strengths rather than just getting the best score with these assessments.
Morrisby assesses your verbal and numerical reasoning (similar to the 11+ exam). However unlike most other assessments, they also include practical aptitudes of abstract (the ability to interpret graphs, data, etc), spatial awareness and mechanical abilities. The latter focuses on the practical skills of making things.
So this is what makes it so accurate in terms of the careers provided.