If the two English translation differences were minuscule like a word or two swapped for something that makes more sense for that region, then that's fine. However if they are reworking the entire translation, then that seems a bit excessive.
They do it to reduce localisation times. Years ago NoE would have to wait for the NoA translation to be done and then work from that. Now with text-heavy games they both get started on their own script while the game is being written in Japanese. Sometimes they still share the US one but more and more they are getting different scripts. All the trophy text in Smash 4 is different for example.
In rare cases like Xenoblade the US version will use the EU localisation, but that's unusual.