Just guessing, but it could simply be that the CRA makes the distinction in it's reporting because CUs tend to look upon a default against one CU as a sin unto all CUs. Default with one CU, and others will likely not extend you any credit. CU blacklisting.
I daresay not all do it, but I'm sure that some will because a CU "is family, not like those evil banks."
Could be tantamount to the CRA saying, "Awwwwwwwwwwwwwwwwwww, I'm telling."