Just because your UAI differs between year to year doesn't mean the calibre of students follow the same trend.
First thing to note is that your UAI is composed of DIFFERENT SUBJECTS. Since your UAI consists of a limited number of subjects it would be wrong to say changes in your selected subjects reflect changes throughout all subjects.
The calibre within each course fluctuates from year to year. So the calibre of course X in year 2005 may be better than the year 2006 but worse than the year 2007, whereas the calibre of course Y in 2005 may be worse than 2006 but better than 2007. You may only do course X and not course Y and only see the effects of course X on your UAI and not course Y.
However, OVERALL the number of band 6s is rising despite some courses decreasing in band 6s in one year and other increasing in the same year.