Why are some met2013 MSAs missing in some years but not others?

I am looking at data from ACS one-year samples from 2006 through 2015. Using met2013 to identify MSAs, there are 56 MSAs in my combined 10-year data set that do not have obervations in all 10 years. For example, Charlottesville, VA and Durham-Chapel Hill, NC are both in there for 2006 through 2011. Columbia, SC has observations from 2012 through 2015. Why are observations from some MSAs not available during all years in my window? I would like to track trends over time, so I’d like to treat MSAs consistently across years.

Since 1990, the only sub-state-level geographic information available in census files is the PUMA. In some cases, the boundaries of PUMAs do not line up perfectly with the boundaries for metropolitan areas. The MET2013 variable uses a standard protocol to identify metropolitan areas, however, in some cases the metropolitan area is unable to be identified. This protocol is detailed in the variable’s description tab.