I imagine it started with some sub-installations actually giving approximations that were acceptable and summed up, but then some finalizing was not taken into account or something needed to be added after the other processes are finished, and the deadline was close. That last part builds up over time with other quick additions and some annoying stuff that is actually quite performance heavy and not easy to incorporate through the whole installation. "Let's do it at the end as well."

No time / budget to change the 100% to 99% as they have to adjust calculations based on the processes that actually do a good job. Although a display change could fake it, priorities are elsewhere.