There are differences between English and US publishers. I'm not sure why but I'm discovering that it can be quite significant at times.
There's William Collins in the UK, Harper and Row in the US and HarperCollins in Australia - all part of News Corp, by the way. There's Mills & Boon in the UK and Harlequin in the US. Etc.
Even books can be published differently. Cross Stitch by Diana Gabaldan is called Outlander in the US. Lee Child's The Visitor is Running Blind in the US.
I think, here in Oz, we mainly get UK editions, which all have "not for sale in the US or Canada" clearly printed on them. I guess it's the same for US publications and sale in the UK.
Strange is the world of big business.