Weird slurs

[Tomas Valusek]

Hello,

in the following piece of code, the slurs should look approximately the 
same - but they don't.

%Begin code
\version 2.8.1
\relative c
{
\clef bass
\key c \major
\time 4/4

c8( d' a d c, d' a d)
c,( des' as des c, des' as des)
}
%End code

How to fix this? I'm absolute beginner in LilyPond and this is a small 
snippet of my first transcription.

Thank you for your help.

Tomas Valusek