Matlab doesn't round-off automatically. You might mean the values shown in the command window. But that's only the displayed value. The actual value in the memory has more digits. You can for instance change the display mode with
to see more digits. In any case, the calculations are performed on all digits available.
If the error indeed lies in the file import, you need to provide much more information than "it's a dat file". That says nothing except that there's data.