Grave Markers are notoriously incorrect. While they can be a valuable source of information, they are never considered a "Primary" source and are trumped by birth, death, and marriage records. Everything on a Grave Marker is dictated by the person handling the burial - so birth and death dates, marriage dates, even names can be different from what is recorded in 'official' records on file with government agencies, churches, etc.
As Alan stated, differences in dates and other information can and should be explained in the Biography section. In this instance, it's unlikely that there are official birth / death certificates. Most of this information appears to come from a book that was written about the family, which I have not read - so I"m not sure of the original sources for these dates.
Perhaps Sue Fitzpatrick, the manager of this profile, will get back to you with a clarification. If you have a photograph of the grave marker, you might offer to up load it.
Happy Holidays!
