So I have a bit of a theory, I mean look at them!
http://i42.tinypic.com/ngufk9.png
They're close relatives that's for sure! But lets take a moment to look at the facts for a second before you fanboi's jump on me.
"
Kabuto is a rock type and Genesect is bug type, that doesn't work "
Right but Kabutops is only rock type from being a fossil look at Kabuto without knowing it's a rock type, you'd probably assume it'd be a bug type, making Kabutop's a Water/Bug before fossilization.
"
It is indicated that even before Team Plasma powered up Genesect, it was a skilled and ruthless hunter. "
Well then...
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Kabutops is a fierce prehistoric predator. Adaptations in its body permit it to lead a more terrestrial lifestyle than its predecessors. It is also very adaptive as shown when its prey started living on land and Kabutops adapted to living on land as well."
Sounds pretty ruthless to me!
Furthermore fish started evolving into land creatures of 400 million years ago, Kabutop's is in the middle of this stage, Genesect is allegedly from 300 million years ago, the stage of evolution would've all ready been complete, thus explaining genesect's inability to swim like Kabutop's.
Knowing that Team Plasma focused on Kabutop's special attacks more than physical, this explains why Kabuto's attack of 115 is only boosted a little bit to 120 while his special attack is risen to this amount by mounting a cannon on its back.
"
But then why does Kabutop's have more physical defense"
Simple. Kabutop's defense comes form its rock-like exoskeleton, rock is a lot more durable than steel, for example, throw a rock at another rock, the rock will probably not break or show signs of damage. Throw the same rock at someones car, its more than likely to dent.
I think I've explained myself enough... Genesect learns rock polish :3