Or it makes perfect sense since in the span of those five years, they are performing exploration work. Perhaps you'd feel better if the statement was amended with, "and returning home periodically to do non-exploration work" ? Is that even true? Did Kirk's five year mission tie him down at all to prevent them from fulfilling their narrated mission goals and still performing routine tasks for starfleet? I think not. That's pretty reasonable. It is only at odds with the five year mission if the ship did none of the tasks set out by the narration. Since it did accomplish all of those tasks and even more mundane tasks while all out in space, "out there", then it is puzzling why you'd claim otherwise.