I've run into an issue analyzing some NHANES data that might be a problem in other datasets as well. xport finds columns with num_decimals == 0 when the data is not an integer. I can't tell from the specification file if the .xpt file is in error here, but I've created a minimal example of the problem so that you can investigate further if you have time.
http://nbviewer.ipython.org/5004964/