This massive collection, now 45,915,080 records, is likely updated as previously embargoed records move into the public realm.
The 1939 Register is a detailed survey of the civilian population in England and Wales during the Second World War, initially taken for ration books and to issue identity cards. It includes notations that give the married name of women originally registered under their maiden name.
Findmypast’s version of the Register lists 36,688,071 records; MyHeritage has 35,119,597 records. Could it be the Ancestry version includes corrections and married names as separate entries?