It might have been deliberate. If the "UG" (in AUG) and "JU" (in JUN) were lower on the leap year subdial, they would practically touch the year numbers "1" and "4." That would look even worse. The only way to make everything line up perfectly would be to move the entire leap year mechanism (and its subdial) down, but then the bottom right portion of the leap year subdial would be too close to the applied gold rectangle at 4 o'clock on the dial. The only way to cure THAT issue would be to make the leap year markers even smaller (in order to shrink the size of the leap year subdial so that it doesn't come too close to the applied gold rectangle at 4 o'clock), but then the leap year numbers would be microscopic. So the slightly misaligned AUG and JUN months might have been the best solution, with the thought that the "step-down" between the month subdial and the leap year subdial would make the misalignment less noticeable. That's just a hypothesis of course. I do think the leap year subdial could be smaller if they just used 3 black and 1 red dots or hashes (rather than the actual numbers 1/2/3/4) to represent years, though. If they used dots or hashes rather than numbers, the year subdial could be miniscule.