tie direction !?

[Thomas Morley]

Hi,

please consider the following example:

\version "2.19.82"

\relative c'' {
  f,1
  r4 b~ b8 a b4~
  b8 a b2.
  \repeat unfold 8 f8
}

The ties have different directions.
If you change the example a little bit you may (or may not) get equal
directions.

Where does it comes from?
Or wit different wording, what are the conditions for the tie's
direction applying here?

Thanks,
  Harm