That's very well spotted for the theme Mackania, and makes the construction by Brendan all the more remarkable. I had started trying to find significance in the preponderance of some letters in the completed grid and had actually counted them all (!) but hadn't made the link. For those like me who like numbers as well as words, there are 137 letters in the grid, two-thirds of which are in down clues (58% excluding 8dn). The characteristic that Mackania has spotted confined Brendan to using just 10 letters in those down clues, and explains why just two letters account for 35% of the completed grid!