That is usually the case because otherwise, you could not get the mattress into the boat. In some cases, the top of the mattress is connected. To remove the mattress, it folds over itself. Functionally similar. Either way, the mattress split is not normally noticeable.

Warren S/V Scuba Doo
Lagoon 50 (winner of best crewed yacht under 55 feet at the St. Thomas crewed yacht boat show).

https://www.cyabrochure.com/ebn/2307/pdyrX/6642/4///