How they decided on these factors lies somewhere in the vaults of ICC. Some reasoning had to be submitted (like it is required now) to make a change. Therefore, some science may have been involved. I am sure fluid mechanics plays a part in the study of crowd movement. There have been several studies on the subject, and many computer modeling applications are based on these studies. As artificial intelligence continues to improve, these computer models will be better at replicating real-life scenarios.
Means of egress is like a highway system during the morning rush hour. Everybody is leaving at about the same time from various communities around the city, all converging into the city. As the number of cars increases, the number of lanes increases. When there is an imbalance (too many cars for the number of lanes), things get backed up, and you have your morning traffic jam. The DoT then acquires funding and adds as many new lanes as it can based on available land and cost.
Buildings are much the same way, except the DoT is usually reactive whereas the building department is proactive. The building department will not allow you to increase the occupant load without increasing the egress width, especially in an existing building. Thus, you are required to make the changes to the means of egress before the occupant load is permitted to be increased with a Certificate of Occupancy.
A Brief History of Egress Width Factors (UBC through IBC)
I have no history with NFPA, so I do not know how they achieved their numbers because my experience with building codes started with the UBC. Before the 1991 UBC, egress width was calculated by dividing the total occupant load by 50. This method goes back to 1955 (the oldest UBC copy I have is the 1955 edition).
The 1991 UBC changed the width calculation to 0.3"/occupant for stairs and 0.2"/occupant for everything else. The 1997 UBC provided a table that modified the widths slightly for Group H and Group I-2. This table was carried over into the 2000 IBC but also added reductions for installing a sprinkler system and remained the status quo until the 2009 IBC.
The 2009 IBC eliminated the sprinkler reductions (which pissed off a few people), but the specific requirements for Group H and I-2 were eliminated. However, those areas were already covered in other parts of the IBC by establishing minimum widths greater than those for common corridors.
The 2012 IBC saw the comeback of the reduced width factors for sprinkler systems but also added the emergency voice/alarm communication system as a required condition. This is the situation as it remains today with the 2021 IBC.