ATA is actually still a 4 digit number just like every other stat, however the 4th digit gets truncated and never shown to the player (this is why things like c/ability are 30 all stat and 3ata, it's technically just +30 all inside the code). A humar at lvl 200 displays 174 ata with no equips, but its actually something like 1748 in the code (so lets say 174.5). As an example, a v101 give 15 all stats, which means it's actually 1.5 ata, so they will appear to gain 2 ata on the stats screen from equipping one, where some other classes will only appear to gain 1 ata (as their hidden digit is too low). Since the stat screen hides that last digit, it's not something that you would intuitively see without actually testing it directly. The practical advantage of having hidden ata is that those classes will gain their ata on the stat screen with odd mag dex levels rather than even ones, so you are able to squeeze out 1 level more than you would think when doing the math for maxing plans.
So yes, exactly, some classes don't need the extra 1 dex on mags so they can use it elsewhere.
Edit: fixed example