In GRIB files, variables are identified by numbers. For example, number 7 is the geopotential height measured in meters. One would have thought that NCEP would use one set of numbers but that would have been too easy (this is the government). Unfortunately the tables used by Operations and the Reanalysis Project have diverged.
If we had known that there were 2 NCEP tables, we would have changed the table identification code. Unfortunately we now have the situation where the operational and reanalysis grib tables can have the same table id. Fortunately the common variables have the same codes; however, you can have problems decoding some of the less common fields.
Please be aware that if you use operational and CDAS/Reanalysis data, you will have to use the corresponding grib table. The pre-v1.6 versions of wgrib use the Reanalysis tables so you have to be aware of the differences between the two grib tables. Version 1.6 of wgrib will include the operational grib table and an option to change between the two tables in cases where the code cannot determine the origin of the data.