Firstly, the Express assessment is not as detailed as the Comprehensive assessment and so the results are not as accurate.
Each applicant's score is mapped against an average score and the standard deviation from that average.
For some applicants, they may have results that are not in the red (amber or even green) but overall their result is dramatically away from the average. This results in a score of zero.
It should be understood that a Role Fit score of zero is quite rare, less than 1:1000 will have a score of Zero.