It's dataminers. I worked with a few. Lists of information are bought and sold frequently. Some of the lists are bits and pieces. Different companies aggregate the information differently.
Consider these totally fictional people. In this case, they are different.
John Smith, SSN 123-45-6789 DOB 01/01/1980
John Smith, DOB 01/01/##
John Smith SSN ###-##-6789
They would all be associated to be the same person. Some list providers only provide partial SSN. Some partial DOB. Sometimes neither, but a physical address.
Some providers consider people who lived at the same address in the same timeframe (+- 5 years) to be related. So if you live in an apartment for a year, in their eyes you're actually related to 8 set so of other tenants. For bill collectors, that's good enough to say you're a source of information..
Phone numbers are a life-long identifier. It doesn't matter that most people have changed phone numbers dozens of times. Again, having the same phone number can tie you in as family..
And sometimes their lists are just plain dirty. I've seen lists by places who take random bits and pieces and stick them together.
It's all fun and games, unless a search warrant is issued for your home, because some list provider tied your address to someone else.