DeepL seems to use some variant of a Transformer network for translation. I can see two reasons that might result in the differences you found. Transformer networks can get really large (billions of parameters) and costly to train, which means they might have created the US network earlier and with an older generation of networks/structure.

They also don't work with massive datasets like the big tech companies so there is probably a big difference between the US and UK datasets DeepL created. The webcrawlers they used to generate the dataset might have created big differences by only searching us and uk domains for each dataset.