Counting only the MSA’s with an “X” in the PWMETRO code list, I see that the IPUMS-USA 2011 ACS sample identifies 293 MSA’s. I get a number closer to 380 (363) only if I also count the codes that are not identified in 2011 (the N/A code plus all codes marked with a “.”). Are you perhaps counting all of these cases? Most of the codes that are not identified om 2011 (those marked with a “.”) are MSA’s that were identified only in older census years (1990 and 1980).
You’ll note that there is still a difference between the 293 codes identified in the 2011 ACS sample and the 280 codes in the ranking table you linked to. That’s because the ranking table lists “consolidated MSAs” (CMSAs), which consist of two or more “primary MSAs” (PMSAs), and IPUMS USA codes identify the component PMSAs separately. E.g., the ranking table identifies “Dallas–Fort Worth, TX CMSA” as a single entity, but PWMETRO codes separately identify “Dallas-Fort Worth, TX” (code 1920) and “Fort Worth-Arlington, TX” (code 1921).
(Note that the IPUMS USA labeling of metro areas can be misleading. In the 2011 sample, PWMETRO code 1920 does _not_ include Fort Worth. Only code 1921 does. For more info, see the discussion of “The METAREA Label System” in the METAREA variable description. We hope to improve this labeling in the future.)
The codes for the METAREA and PWMETRO variables should all align with each other correctly (i.e., a single metro area will have the same code in both variables), but we are able to identify a somewhat larger set of codes for METAREA than for PWMETRO. That’s because IPUMS USA determines METAREA (the metro area of residence) based on Public Use Microdata Areas (PUMAs) and determines PWMETRO (the place-of-work metro area) based on Place-of-Work PUMAs (PWPUMA00). Some Place-of-Work PUMAs are larger than standard PUMAs, and they may straddle metro area boundaries in different ways, so it is not possible to identify as many place-of-work metro areas as metro areas of residence.
To understand the relationships between IPUMS USA codes and the original Census/OMB codes, see the discussion of “The METAREA Code System” in the METAREA variable description. In general, each IPUMS USA code corresponds directly to a single MSA or PMSA in Census sources, but the codes may differ.