I'm not familiar with US printing standards, but it may be a matter of different data sets - in Europe we have different characterization data sets for standarized printing conditions, created with small (IT 8/7.3 918p), medium (ECI2002 1485p) and large (IT8/7.4 1617p) CMYK target. So for example - there are two sets of characterization data for CMYK sheetfed offset, paper class 1&2(coated), periodic screening - FOGRA39 (1485p), and FOGRA39L (1617p), and the different placement of color paches in both targets may result in slight gamut shape differencies.