Once upon a time, the game was showing different values on the in-game interactions interface.
Consistent with the “percentage” that was stated on the wiki about each epic arc.
After an update (long time ago, I wasn’t playing then), the in-game interactions interface that shows the standing gains, it started showing the values differently, divided by 10 (or multiplied by 10, I don’t know for sure, I haven’t got any screenshots from back then).
For example, the epic arc on wiki shows that you will get 12.5% faction standing, but the in-game interface will show you a +1.25 (and you actually get that 1.25 if you do the math).
So, all the info you see on wiki or other old posts regarding percentages, you will get that value divided by 10 and not 100 (to transform a percentage to the actual value you divide by 100).
That is why my graph shows the 1.25, not as a percentage, but what you will actually get and you can use on the equations to predict your future standings.
After that, the equation of the diminished standing gains depending on your base standing before the gain takes effect and you actually get a different standing gain.
More info about the equations used: