As part of my employment I have access to a number of these commercial databases, and I'm surprised that's all. Typically every husband/wife will be "associated" with each other's SSN. Likewise, depending on the algorithms used by the data provider, linkages may also show for co-signers of loans, joint property owners, etc. That's not to say there aren't people out there misusing other's SSNs, but simply saying one person is "associated" with more than one SSN should not, in and of itself, be considered any kind of revelation.
Exactly. If a husband dies young and leaves behind four kids, then six people are then 'associated' with that number because they can draw benefits off that earnings record.
You also see multiple names associated with one number due to marriages and divorces - most women will at least have two names associated with their number (maiden and married names). Additionally, minor variances could appear as multiple names. It would not be uncommon at all to see Robert Jones, Robert A Jones, Robert Andrew Jones, Bob Jones, and Bob A Jones all listed on the same number.