The calculator rounded the answer.
The exact value of [tex] \frac{2}{3} [/tex] is [tex] 0.\overline{6} [/tex], i.e. a zero followed by infinitely many 6s.
The calculator can store only 4 decimal digits, so it has to approximate the number. By doing so, it has to "choose" between 0.6666 and 0.6667.
Since the next digit is a 6, the approximation is 0.6667.
In fact, if you have to approximate with four decimal digits, you have to look at the fifth. If it is between 0 and 4, you use your fourth digit. For example, 0.35742 is approximated by 0.3574.
If instead the fifth digit is between 5 and 9, you raise the fourth digit by one: 0.57836 is approximated by 0.5784
