There has been much debate on discrepancies found between Census generated and locally generated intercensal county estimates.  Numerous anecdotal speculations on the cause of the discrepancies at the state level have been made.  A more systematic study by Davis (1994) assessed the accuracy of the Census Bureau’s county population estimates by examining the results of the Census Bureau’s estimation methods against the 1990 Census count.  In this study, we examine the Census Bureau’s Administrative Records method and hypothesize that there are systematic biases in the records used in the estimation method that are the source of the discrepancies. We examine, in detail, the sources of data used in the Census Bureau’s methodology.  Based on this examination, we develop a theory explaining why there might be a systematic bias in the administrative records themselves and in the data collection process itself. We test for these biases by using county level indicative economic and demographic data. The theory identifies causes of discrepancies in estimates that are systematic to the methodology and suggest the direction and likely magnitude of the discrepancy. In virtually all cases, our results are completely consistent with a priori hypotheses.  Furthermore, they remain even when undercount adjustment is made.  We conclude by presenting several recommendations, including incorporating an adjustment factor, improving vital statistics geocoding, improving group quarters reporting, and testing for medicare undercoverage.