Not that this thread takes the lore seriously for the most part, but in the amazing novel Darth Plagueis novel (seriously one of the best SW books I've read, including the Thrawn trilogy), Palpatine's background and origins are explored much more.
His first name is indeed Sheev since that was revealed in a previous book by another author, but he resented his family and rejected everything about them, including the name Palpatine, in his youth. He eventually met Darth Plagueis (not yet knowing he was a Sith Lord) and as he grew up and some events happened that I'll not spoil, he eventually came to embrace the heritage of his family name but rejected still his given name (as he deeply hated his father who had chosen the name).
So that's why he's only ever called "Palpatine" outside of that one book where Tarkin calls him Sheev (which would probably call for getting his wrinkly ass sith-lightning'ed considering how much Palpatine despises that name outside of that book).
Read Darth Plagueis, it even makes the midichlorians tolerable no joke.