Curious thing about ties

[Robert Schmaus]

Dear Ponderers,

something I just came across while pondering away: in the following 
snippet, there are two ties across barlines.

%%%%%%%%%%%%%%%%%%%%%%%%%

\version "2.19.35"

\score {
  \relative c'' {

    | a2 r8 a8 ( a ) b ~
    | b2 r8 b8 ( a ) b ~
    | b2 r2
  }
}

%%%%%%%%%%%%%%%%%%%%%%%%%

I've wondered why in the first instance, the tie is a down-tie, while in 
the second it's an up-tie (see attached screenshot). Is there a reason 
for this? Something like visual balance?

I'm running Lilypond on a Mac. And I should add, that I'm really just 
curious, I don't have a problem with this behaviour.
I'm guessing that this can only occur with notes inhabiting the middle 
staff line, right?

Anyway, take care,
Robert

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