There seems to be a natural tendency in language for the first few terms of a sequence to be treated specially. Thus in English, "eleven" is really "one left (over)", and "twelve" is "twa left", so that the implied "after ten" is omitted. But from thirteen on it seems that we should explicitly mention the "ten", since we're getting so far away from it that we might otherwise forget it!
The break after 60 in many languages is even more interesting. However, I have to teach a class now, so must dash.