If you're using smilies, I think that can change how many "characters" you have available in a text message. I see 70 if I have a smiley in there, but 160 if I don't.
Setting > Network+ > SMS character set
If it is set to full, it will allow foreign languages but reducing the length of the SMS by half.
If you want 140 characters, simply set it to reduced.
What's the rationale behind it? Why was it introduced in Lumias?