As noted above, the CMIP5 archive is evolving and in some cases the data used in the IPCC 5th Assessment Report may have been superseded.
The DDC Reference snapshot is more extensive than the WGI snapshot: It contains data that was discussed in the scientific literature and thus contributed indirectly to the IPCC 5th Assessment Report.
The reference snapshot is also based on the status of the CMIP5 data archive as of March 15, 2013.
The IPCC's Fifth Assessment Report (AR5) relies heavily on the Coupled Model Intercomparison Project, Phase 5 (CMIP5), a collaborative climate modelling process coordinated by the World Climate Research Programme (WCRP).
The CMIP5 archive has evolved through and beyond the IPCC 5th Assessment process, with modelling groups eager to contribute their best available data to the research community.