It's because some of his stories are out of copyright in the US because of the arcane registration requirements that used to be in force.
Some of those stories might also be out of copyright in the UK, but determining that is even harder, because it depends on exactly when they came out of copyright in the US.
I wish every nation would agree on something simple, like the greater of 50 years after publication and the lifetime of the author.
But that's not going to happen. But we could get everyone to agree on lifetime+50 years, which is the Berne convention minimum.