That is the "Cumulative Affinity Requirement" to get to rank 40. There is no way to get to rank 40 without earning 3,710,000 affinity.
Put another way, no one would say it only took 450,000 affinity to have a weapon with one forma. They would say it took 900,000 affinity. I'm saying the same thing except accounting for the higher levels.
That thinking is technically not wrong, the table does say "Cumulative" after all. But one could argue cumulative refers to the amount of affinity needed per Forma cycle, not grand total. Or rather that a per-Forma-cycle definition would be more useful and easier to understand than a grand-total definition.
I think the simplest solution is just a change in title. I.e. instead of "Cumulative Affinity Requirements" it should maybe say "Total Affinity from Rank 0 Requirements". An actual cumulative total chart might cause confusion and be a presentation of info that could be easily misinterpreted.
For now, I changed the title of the chart to better reflect what it is actually showcasing. You are right with the fact that the cumulative affinity needed is more than what's shown due to the fact you have Forma those weapons and re-level them up multiple times.
Thanks for the heads up. Maybe in the future I'll look into adding more info on the true cumulative affinity needed.
They would say it took 900,000 affinity. [...] Am I wrong?
On this point, yes. No one really talks about how much it took in total to level a weapon to a certain forma-state. How much affinity you need to collect, to get to max rank in a single mission is more relevant, however.
Also the growth of Affinity required per level is more easily gleaned from the current values. Meanwhile what you propose has little implications and can easily be calculated from provided values. The other way around theoretically too, practically not.
So yeah, I'm in favor of keeping current values and not adding any additional ones, as they are likely to just confuse people. Personally I would just integrate them into the 0-30 table and make it 0-40 with a note that only few exceptions can pass 30 that mark