The proportion of Welsh candidates getting A grades has increased by nearly three times the national improvement rate because modular exams allow them to retake parts of the course to improve their performance, according to the Welsh Joint Education Committee.
Previous research has suggested that modular exams increase the overall pass rate but make it harder for candidates to achieve the highest grades.
However, the overall pass rate for this year's A-level candidates in Wales increased by just 0.3 percentage points, compared with a leap of two percentage points for A grades.
A spokesman for the committee said: "One contributory cause of the increases in these proportions must be that there has been an increase in the entries for modular exams.
"This 'staged' assessment increases students' motivation and allows candidates to repeat modules to improve their performance."
Welsh sixth-formers, like those in England and Northern Ireland, achieved their best-ever A-level results this year, with nearly 93 per cent of candidates getting at least an E grade.
The proportion of A grades rose from 17.3 to 19.3 per cent, compared with 17.5 per cent nationally. Although the Welsh board pass rate increased at less than half the national rate, at 92.8 per cent it remains above the national rate of 88.5 per cent.
The most significant increases in pass rates were found in design and technology (up 3 percentage points), chemistry (up 2.5 points), biology (2.1 points) and English literature (up 1.7 points).