Perhaps because they weren't on a quadrant mapping mission.
They were just looking at and scanning specific planets that might meet the requirements that Dr. Marcus had laid out for them to do.
One could surmise that they had been at it for quite some time, due to the conversation that Chekov had with Captain Terrell which sounded like they were both rather exasperated by that point.
Meaning that they may have entered the system only hardheartedly paying attention and just focused in on what appeared to be a possible planetary candidate.

They were just looking at and scanning specific planets that might meet the requirements that Dr. Marcus had laid out for them to do.
One could surmise that they had been at it for quite some time, due to the conversation that Chekov had with Captain Terrell which sounded like they were both rather exasperated by that point.
Meaning that they may have entered the system only hardheartedly paying attention and just focused in on what appeared to be a possible planetary candidate.
