"...ask them how the got their numbers." I can see how they go...

"...ask them how the got their numbers."

I can see how they go their numbers. I don't need to ask them
The say so in the very first chapter entitled "Data".
Which, self-evidently, you didn't even bother to read.
(Your own source, but you don't even read it or properly understand it.)

As I suspected, the data is merely generated by numerical simulation using a semi-empirical model. It makes a host of assumptions, including that flux changes are magnetic field induced. Which itself is an approximation.

But then we need to know what the earth's magnetic field has done in that case, which we also don't know.

In short, they got their numbers from models.
But models are just that, estimates.
A chain is only as strong as its weakest link, and a numerical model is only as accurate as its least accurate assumption.

And the nature of the situation here is that they least accurate assumption is not even accurate to a few percentage points, let alone accurate to 0.03%, which is what the chart which you posted, presented.

And you have the temerity to accuse others of not contributing scientifically rigorous manner?

.