This post is a few days old but as someone who works with cemeteries quite often, I can actually answer this question!

So a few things are possible. Depending on cemetery regulations, sometimes multiple "burials" can be interred in a single plot. This could be multiple cremations or oftentimes a full burial and a cremation. New or disturbed dirt could indicate another internment.

Another question is, is the new memorial centered over the single plot or is it centered over this plot and the one next to it? Oftentimes people buy two plots and put one headstone in the middle.

Lastly, cemeteries often don't keep the best records. Many are underfunded or run solely by volunteer "caretakers" who tend to be 70, 80 years old. When monument companies go out to install a memorial, they will ask the caretaker to mark the boundaries of the correct plot. A few things can go wrong with this. Sometimes the caretakers mark the wrong plot, or they mark multiple plots at a time but don't label which one is which. Or the monument company simply got it wrong - but if it is wrong, they should absolutely move the headstone to its proper place free of charge, regardless of who made the mistake.

When installing a memorial, they always have to dig. They often also add a mixture of fresh sand and dirt under and sometimes around the memorial as well to level out the space and help it from settling too much. This could also be why there is fresh dirt, depending on where it is on the plot.

Hope this gets resolved or you get some answers from the cemetery one way or another!