Why are the Pokemon in the "Fluctuating" EXP Group actually there?

Question

http://bulbapedia.bulbagarden.net/wiki/Experience

If you check the Pokemon located in the group, you see it contains Pokemon such as Seviper, Makuhita, and Breloom. Being that this group has the highest EXP total, shouldn't Legendary/Extremely rare Pokemon be here? That Q brings me back to the original Q, so why?

Comments

I really think this is just a stupid function. #Gamefreaklogic

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I have done much research and failed to find any reason why Gamefreak added the fluctuating exp. group. I have found rumors of it being for balancing pokemon and for more variation, but no definative answer was found. I am sorry but it maybe that you will have to ask Gamefreak itself. I hope this helps your hunt.
DracoArceus

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Draco has a nice comment there.
It was probably added for more variation in level ups, and it makes sense in a way.

Answer 1

There is no particular reason for it, from what I can tell here are the similarities.

Most are from Gen III: Driftblim/floon are the exceptions
4 families have high HP: Wailord, Hairyama, Swalot, and Driftblim
Aside from Seviper and Illumise they all are part of 2 part evolutionary lines.
None are legends as you said.
Most have uncommon base stat totals.

But it seems that they were random Pokemon were dumped into it. I will look into it a little more.

EDIT: I have found a potential reason, many of these Pokemon are not particularly hard to get at low levels in the game (correct me if I am wrong), so they can level up fast to lower levels to serve an ingame purpose, also most evolve in the quick leveling up part(or near it) of their fluctuation. Legends on the other hand are strong and are found at high levels.