Since your results were discrepant from the existing body of literature, it would be virtually impossible to provide concrete answers/explanations for the disparity, as the first tutor indicated.
However, the following variables could be mentioned as possibilities which necessitate further research to confirm:
-Level of stress and personality type (ie, a very high-achieving student, particularly a Type-A Personality which high-achievers preferentially are, is far more likely to feel chronic stress and anxiety during college than a lower-achieving student with a Type-B Personality, for whom the unattainable ideal of perfectionism isn't a goal).
Research indicates Type-A personalities do not feel any joy from completing their goals, as they are highly self-critical. The lack of joy and/or sense of accomplishment subsequent to completion of a goal could plausibly account for the negative correlation you observed.
-Quantity and quality of social interactions (Ie, a very studious individual is arguably far less likely to spend a significant amount of time socializing outside of the classroom given the time dedicated to studying, and we know from existing research that social bonds contribute substantially to happiness, particularly when in novel or stressful scenarios)
-Time period of study (ie, if this was conducted during the past year, whilst the COVID pandemic substantially changed the college experience for students, then the novelty of the pandemic could very well have contributed to the disparity between your results and what you found during your lit review).
I would imagine that high-achieving students have been particularly negatively impacted by the pandemic, since that personality type necessitates a predictable set schedule/curriculum to eliminate the element of uncertainty and consequent feeling of unpreparedness, which exacerbates stress levels.
Finally, the lifestyle habits of high- vs low-achieving students should be considered. The former is arguably more prone to sleep deprivation, (which we know significantly raises levels of cortisol and wreaks havoc on mood regulation, executive functioning, etc), given the time dedicated to schoolwork, while the latter is more likely to prioritize sleep and/or relaxation over grades.
Here is one interesting full-text study about personality types associated with academic performance and respective levels of coping:
Best of luck with your dissertation!