Andy,
The reason is that different countries have different copyright laws.
Most of the books you mention (Wodehouse, etc) are in the public domain in the US, but not elsewhere in the world. This is because any book published prior to 1923 is PD in the US, regardless of when the author died. The US now has a "life + 70" copyright law, but this only applies to books published after 1978.
I don't know why "Animal Farm" should be available from Google. It is not in the public domain either in the US or in any "life + 70" country. It is in the public domain in Australia, where any book published by an author who died prior to 1954 is PD. Australia used to have a "Life + 50" copyright law, but switched to "Life + 70" in 2004. Any books which had already entered the public domain at that point, however, remained there.
Hope that's some help to you.
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