Every student who enrols in a degree programme involving service mathematics in the University of Limerick in Ireland is given a mathematics diagnostic test. The diagnostic test was developed due to mathematics lecturers' anxiety regarding students' mathematical competency levels. Students receive the 40 question paper-based test in their first service mathematics lecture without prior notification. Initial analysis of students' work revealed that many students were having difficulties with basic algebra and arithmetic in particular [Gill, O. 2006. "What Counts as Service Mathematics? An Investigation into the 'Mathematics Problem' in Ireland." PhD diss., University of Limerick]. Further research highlighted the significant decline in diagnostic test performance and the changing profile of service mathematics students between 1998 and 2008 [Faulkner, Fiona, Ailish Hannigan, and Olivia Gill. 2010. "Trends in the Mathematical Competency of University Entrants in Ireland by Leaving Certificate Mathematics Grade." "Teaching Mathematics and Its Applications" 29 (2): 76-93]. One of the most notable changes to the student profile over time was the increase in mature students (non-standard students) entering service mathematics programmes. Although non-standard students had a lower mean performance in the diagnostic test, they were found to have a higher mean performance in some cases in service mathematics compared to standard students. This paper explores some of the possible reasons for such findings.