Just to let you know I agree with all the above as I have had a similar experience. My great, great uncle died unexpectedly in his twenties after a motorcycle accident. He is buried with two other seemingly unconnected people and the headstone only refers to one of them. My great great gf is buried in a family grave with his descendents and the other siblings are in other family graves with their respective partners etc. I was mystified until I noticed that his wife's death (who died much later and is not buried with him) was registered by her sister (death cert), whose married name is the same as one of the people in his grave. Therefore, I concluded that the grave belonged to his wife's sister's inlaws (hope you are following this). A bit complicated but it does finally make some sort of sense.
Another thing to watch is where the names on the headstone don't follow chronologically with when they were buried. I have noticed on two of my stones that the children, who died and were buried before their parents, appear after their parents' names ("and their beloved children..."). I am assuming that they could not initially afford a stone and that this was erected much later and the names inscribed retropectively.
Also, where you have two family graves next to each other, bodies may have been buried in one grave and their name appears on the other headstone. For example, a child's name appears on the stone with it's parents, but was actually buried in the next grave with the grandparents. However, the council burial records will tell exactly who is buried where and you should be able to account for them all, even if they are split between graves.
Good luck.
Lesley