Meaning that sometimes the ship survived but not most of the crew, or most of the crew survived but lost the ship -- and implying that ships that weren't assigned to 5-year missions aren't included.
Or even that their missions may have been curtailed before the five years were up. The Enterprise, after all, was often reassigned to short term defensive tasks before returning to exploration. Perhaps other ships had more permanent missions?
"Hmm, the Romulans have increased their strength along the Neutral Zone. Better take a couple of Heavy Cruisers off their current deployment and add them to the defence fleet."