Greg responds...

I've answered this a lot.

Do you want the in-Universe answer or the behind-the-scenes answer?

In Universe, Hudson had the most association with humans (i.e. Scottish humans). The others did not and therefore had a more neutral accent. If you listen carefully, you'll here that the trio have a more classical sound to their voices in the tenth century. They were just quick to pick up American idioms in the present. Goliath and Hudson, being older, remained consistent.

Behind-the-scenes, we wanted Hudson to be emblematic of the past. A connection to our characters heritage in Scotland.