I have seen a few zoning ordinances where estate residential zoning district(s) have been separated from other types of residential districts (single-family, multi-family, and so on). In same instances, rural residential is grouped under single-family, multi-family, but estate-residential is a separate chapter.

If estate "residential" is a residential district, why do some ordinances place them in separate chapters? Is it because of the purpose of the district, permitted/conditional/special uses allowed in the district, or a combination of both?
I imagine if estate residential permits equestrian uses (barns, corales, trails on the sides of roads) then this is a different type of use from residential. What are other reasons why this type of zoning district is often separated from other residential districts?