Why do Pauline's parents and the boarders call her Evelyn?

Actually it was "Yvonne" which was her middle name. That's what her family called her at home. All her letters home from prison were signed Yvonne.

Apparently that happens with some people, though I don't understand it. Several times throughout my school years I remember students, on the first day of class, letting teachers know that they went by their middle names.

Obviously Pauline went by her first name outside of her home.

Yes, the names were bewildering. She was Yvonne at home and Pauline or Paul at school. In addition to those were the nicknames that Juliet used: Charles and Gina.

Finally, there was the issue with her last name when it was discovered that Herbert Rieper had never married Honora Parker because he was still legally married to the first wife whom he'd abandoned to run away with Nora.

Nobody called her Evelyn.

And you shouldn't be, since it's stated in dialog the first time Juliet comes over that Pauline's middle name is Yvonne.

HONORA
Eat up, Yvonne.

She leaves the room. Juliet raises her eyebrows at Pauline, who looks
embarrassed.

PAULINE
(Quietly) It's my middle name.

The new home of Welcome to Planet Bob: http://kingofbob.blogspot.ca/