I did some comparisons and exploration, and this is my conclusion of what you get with a purple kit, vs a blue or green kit.
First of all, the rarer the kit, more of the abilities will be II or III grade. Purple kits have all four abilities be III.
Second of all, on rarer kits, some abilities are stronger. Examples would be medical tricorder. Medical tricorder III on a green kit heals a base amount of 247 HP. Medical Tricorder III on a Purple kit heals a base amount of 320 HP.
shame its not always the case i got a blue disruptor item for adding damage to disruptor weps the + ones the one i bought with explore tickets was +30 disruptors weps the blue one i got from the borg in infected space mission was +30 might be a bugged item but totally pointless blue if its no better than the green.
Comments
Because it's an indication of quality and not rarity. Whoever decided that it should be called rarity should be shot. In a friendly way of course.