Three digit figures never get centred in the same way as 1 & 2-digits... I'm not really sure why that is, but there we go.
If I had to make a guess, I'd say it's probably due to compability issues. Some maps may not have been made to accommodate 3 digits, so they may not have space on the left side of the troop circle, or if the numbers would go left of the circle, they would go over something such as territory names or other important stuff. Thus, all numbers beyond 2 digits are printed on the right side of the troop circle. You could argue that not all maps have room on the right side either, but if the 3-digit numbers were centered, then you would need space on both sides of the troop circle.
Besides, some might argue that when you have troop circles sized for 2 digits, it looks better to have at least 2/3 of the numbers inside the circle. Instead of 1 in the circle, and 2 halfway out...