The reason for Monarch's A300 and DC-10 both seating 361 was down to the age of the aircraft. The A300-600R is more modern and makes better use of galley, storage and toilet space. Monarch obtained all their A300s new from Airbus, outfitting them with only front and rear galleys and toilets, with nothing but seats in between.
The older DC-10 makes less efficient use of space, and requires mid-cabin galleys and toilets to provide full service for a high-density layout, consequently the number of seats is limited to below the maximum 380, plus the G-DMCA was configured for long-haul use while the A300 is configured for short-haul services with a tighter seat pitch.
View Large View Medium
Photo © Craig Hendley
As an aside G-DMCA is still at Manchester awaiting it's fate.
Only MyTravel's DC-10-10s seat 380, the sole DC-10-30 (G-BYDA) seats 356 as far as I'm aware...