ok - i found a hint:
Do SMS become more expensive through using special characters?
Answer:
Some special characters in an SMS result in the device changing to a 16-bit code without the user noticing, which reduces the possible number of characters per SMS to 70.
The following are problematic:
- All emoticons (special form of smileys)
- Various other scripts, such as Hebrew, Arabic or Cyrillic
- The following SMS characters:  â, Á á, à ã, Ᾱ ᾱ, Ç ç , Č č, Ć ć, Ê ê, Ë ë, Ė ė, Î î, Í í, ń, Ô ô, Ó ó, Õ õ, Œ œ, Ō ō, Ś ś, Š š, Û û, Ū ū, Ӱ ӱ
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Message size
Transmission of short messages between the SMSC and the handset is done whenever using the
Mobile Application Part (MAP) of the
SS7 protocol.
[39] Messages are sent with the MAP MO- and MT-ForwardSM operations, whose payload length is limited by the constraints of the signaling protocol to precisely 140
octets (140 octets * 8 bits / octet = 1120 bits). Short messages can be encoded using a variety of alphabets: the default
GSM 7-bit alphabet, the 8-
bit data alphabet, and the 16-bit
UCS-2 alphabet.
[40] Depending on which alphabet the subscriber has configured in the handset, this leads to the maximum individual short message sizes of 160 7-
bit characters, 140 8-bit characters, or 70 16-bit characters.
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so i've tested the message without any of these characters ( also no äöüß and so on ) - no change in Result! 
