To explain it as simply as possible:
HttpUtility.UrlPathEncode("http://www.foo.com/a b/?eggs=ham&bacon=1")
becomes
http://www.foo.com/a%20b/?eggs=ham&bacon=1
and
HttpUtility.UrlEncode("http://www.foo.com/a b/?eggs=ham&bacon=1")
becomes
http%3a%2f%2fwww.foo.com%2fa+b%2f%3feggs%3dham%26bacon%3d1