I can't really think of a good thematic reason for the dune scorpion/deathstalker having 0 attack, though of course there are game balance reasons. Maybe they need to be touched by magic to 'awaken' their full potential, or something? :p The forest scorpion has the weakest of the 3 (1 poison, versus 2 poison or 1 neurotoxin) and you could also ask the same question of pufferfish.
For the nymphs, I would suppose that they are highly magical beings and all that, so their power/toughness would vary quite a bit based on their ability and element and so on. For example, the aether nymph, having an ability that isn't particularly useful in monoaether, has the highest attack value of any nymph. The gravity nymph is the most fragile nymph because it is the most powerful one. The life one has a higher power/toughness than most nymphs because life is all about... life and creatures. Light is about healing and damage prevention, so that nymph has more hp than all of the other ones. Not all of it makes sense, but for most of the nymphs, I think their power/toughness + ability/cost are taken into consideration together, are reasonable. I think that the time one doesn't need to be as big as it is given that it has one of the better abilities, and has the 2nd best attack and tied for best hp of the nymphs with light. I think the death nymph would be better suited if it had a higher attack and less hp, like maybe 3/4 // 5/4, and it would make sense for the earth one to have more hp (0/5 // 0/7 like the death one now) but that's just me..