Midgardsomr and the Sundering (Spoilers)

[Seraphor]

Why would they have developed differently? The split created identical copies...

No it didn't. Each shard had a different elemental alignment, or rather, each had a different element with a majority influence.
Elemental aether has an effect on the world around it. A little extra water aether and a certain creature might get a drink somewhere it wouldn't. A little extra fire aether and a fire spreads a little further than it would have done. So on and so on, butterfly effect.

[YianKutku]

No it didn't. Each shard had a different elemental alignment, or rather, each had a different element with a majority influence.
Elemental aether has an effect on the world around it. A little extra water aether and a certain creature might get a drink somewhere it wouldn't. A little extra fire aether and a fire spreads a little further than it would have done. So on and so on, butterfly effect.

Pedantically, when the split happened, the shards were identical.

The differences in aether happened after the Ascians tried and failed to Rejoin the Thirteenth, which caused the Flood of Darkness, and also threw the aetherial balance of every other shard off. (Through some as yet unexplained mechanism where the aether of one shard affects others, which is also how Rejoinings and Calamities work.) We don't currently know if Midgardsormr came to the Source before or after the Thirteenth fell to Darkness.