Why are reluctant learners better at self-evaluation?

The school year is ending, and we're completing our final self-evaluations for a report card grade. Throughout the marking period, I conference with students about their activities and projects and leave plenty of narrative feedback. There are no number or letter grades. At the end of the grading period, I ask students to complete a self-evaluation, reviewing their work and assigning an appropriate report card grade.

Although most students come remarkably close to the grade I would have provided, some are so conditioned to the value of grades that they overvalue their work and give themselves inaccurate grades.

Surprisingly, my most reluctant learners almost never do this.

They are the most honest, when it comes to self-evaluation, while honors-level students are the ones who may tend to "stretch" the grade. There is something strange about this. Most people who know I allow my students to grad themselves believe that students who have suffered through years  of D's and F's are the first to give themselves A's and B's. This is not the case at all, according to my own experience.

So, why are reluctant learners better at self-evaluation.