Why do 2010 shapefiles not align with 2000 and older shapefiles?

Why do 2010 shapefiles not align with 2000 and older shapefiles?

Recent releases of the Census TIGER/Line Shapefiles have included numerous improvements in spatial accuracy (from a greater use of GPS and localized data sources). This has resulted in numerous misalignments between the new data and older, less accurate NHGIS boundaries. To address this issue, NHGIS staff are currently completing a systematic realignment of our historical shapefile boundaries to accord with new TIGER/Line data.