In many cemeteries, if the plot for the first burial wasn’t purchased, the same plot would be used for a future burial, often unrelated.
Yes, my Gt. Uncle (buried second, on top) shares a grave with a stranger.
My Gt Aunt was swayed by her Brother in grief, as it made the cost of the grave cheaper. However, my Gt Aunt who could have purchased a single grave for two in the Council Cemetery, deeply regretted the decision afterward, as she knew she couldn't be buried there. However, her ashes were let in the surface, as requested in her Will.
Unless, it is a Vault, or Family Grave, the Parish Church often buried the deceased with or just above the previous burial.
A couple of times I've found a burial I wasn't expecting, in a "Family grave" - one proved to be the child of young neighbours of the family, in another a relative of the later( keep up) husband of a second wife, who had been widowed ...oddly enough the original husband was parked elsewhere??
You never know, there may be a connection turns up in time....
Anything might turn up, in burial Records and on Memorials.
----------
Not everyone had a Headstone. Others have crumbled, been damaged and/or been removed.
Sometimes, the Headstone is left to the last family burial in the grave and the family alive then (if any), sometimes don't bother to purchase one, or cannot afford one.
----------
Traysha, have you checked to see whether they are distantly related?
Mark