Probably because after his death your father-in-law's mail was forwarded to your husband so that he could attend to the estate, and the postal service provides "forwarding" addresses willy-nilly to clients who request them! You can often see the requests for these on envelopes that come in your mail.
One of my daughters had a letter offering a credit card to "Pupper her last name" recently. Now "Pupper" is her 2 year-old dog, who came by that name when the 4 family members could not come to a unanimous agreement on a name and they realized, after several months, that he thought his name was Pupper, which was supposed to be his temporary designation only. They gave up searching for another name.
We really scratched our brain cells trying to figure that one out! My daughter, of course, called the card company in indignation. The clerk she spoke with thought it was a hoot. FINALLY we figured it out. Another daughter, who has a one year old puppy of the same breed but who lives in Missouri rather than Maryland, thought it would be cute to have HER puppy send her sister's puppy a birthday present last spring. So she ordered a cute dog toy ON LINE from a pet supply place and had it sent to "Pupper my other daughter's last name"! They sell their mailing lists, obviously!
I am in on a 4-member family cell phone plan with one of my sons. Mail for him, his wife, and his daughter come to my home on a regular basis, and NONE of them even live in my state! Nor do I pay the cell phone bill! It goes to my son and he pays it! Go figure! Oh, and no, none of them has EVER lived here!
What an age we live in!
The thing that makes me most angry is medical practices selling their patient lists. I thought doctors were supposed to protect our privacy. Not so! I went to an audiologist last year for the first time. Have not been back there, but Ever Since I am literally deluged in my mailbox with offers of hearing aids and free hearing tests and meetings re hearing problems and You Name It, from dozens of different companies. I find this unacceptable, but am helpless to do anything about it!