You cannot see the full stats from that picture, you need to go to the source (export_descr_units).

Any missile attack has no bonus from experience, only from weapon quality bonus, so in this case it's at 3, despite what the screen says. They also have a very short effective range (thrown stone) with skirmish mode, meaning they rarely get to use their missile weapon at all unless you ignore them. The attack rating and melee defence does get a bonus from experience, however their extremely low lethality means they are still no threat to any regular soldier. When testing their stats, I attacked a garrison of 5 such units with a single sword+shield unit (can't recall which) and only barely lost the battle, killing more than four times the number of men lost even though they surrounded my unit from the start. They are suitably useless.