I'm not asking if this is kind of variation is common or not, I'm asking why exactly InSight's arrival date is pinned down. Are there aspects of the mission that require this? Coordination with other spacecraft perhaps?
About InSight's Launch
InSight is scheduled to launch under pre-dawn skies from Vandenberg Air Force Base on the central coast of California in May 2018.
The mission's launch period is May 5 through June 8, 2018, with daily launch windows that last two hours per day. Launch opportunities are set five minutes apart during each date's launch window. The first opportunity begins at approximately 4:00 a.m. Pacific Standard Time on May 5.
InSight will launch from Launch Complex 3 and ride atop an Atlas V-401 rocket provided by United Launch Alliance, Centennial Colorado, a joint venture of Boeing Co. and Lockheed Martin Corp.
The Atlas V is one of the biggest rockets available for interplanetary flight. This is the same type of rocket that launched the Mars Reconnaissance Orbiter in 2005.
The launch is only the beginning; the trip to Mars takes about six months. The journey is about 301 million miles (485 million kilometers).
No matter at what particular time and date InSight launches during its launch windows, its date with Mars is set for Nov. 26, 2018.