This is because each output has negative offset due to the way device was trimmed as shown in the table below. According to Note 3 of the datasheet (rev 2 5/13), "The device is trimmed such that offset error is negative, ensuring linear operation down to zero current output." For example, the typical offset error of OP1 is -60uA with a full scale of 10mA. The DAC has a resolution of 14 bits. Converting to DAC code, 60uA translates to an input code of 60uA/(10mA) * 2^14 = 98. This explains why the output is 0 for code below 100.