A reasonable explanation...
"While the two are today, the same in size, Sedans and Saloons were once quite different. A Saloon was a larger car, not quite as large as a limousine, but every bit as luxurious. Typically, a Saloon often was Chauffeur driven, while a Sedan was driven by it's owner. Sedans in the 1930s were generally a smaller car than a Saloon, but after ww2 the manufacturers in the US began building much larger automobiles; when these hit the British market, they were often refereed to as "Yank Tanks". Saloon today, implies a more luxurious vehicle; A jaguar S-Type 4.2 would be regarded as a Saloon, whereas a similar sized Honda would be a Sedan."
https://www.cargurus.com/Cars/Discussion-ds444601