My guess would be that the easement was put in with the sale of the property. We have a similar situation and when-if we sell the front property we will stipulate the easement when the property gets sold. Actually where we live the county will no longer allow a land locked parcel
to be sold. An easement has to be granted. For us we would want a larger than normal easement along with the easement that already exists.
If you are keeping A and selling B, but your access to the public road is across B, wouldn't you reserve an easement so you could get to the public road?