Some restrictions in the calculation could come from not including black market labor, workers under or over the working age limits, and hours worked. The employment ratio is also subject to some manipulation by reporting agencies as well, and may be skewed in order to give the perception that an economy, based on this metric, is performing better then it really is.
For example, the numerator does not account for individuals who are currently enrolled in an educational institution and not working. These students are unemployed by choice but it still results in a lower calculation.