Each company has it's very own standards to what a width is.
Back in the Day.....Parker made fatter nibs than Sheaffer, in either that was what their customers at the fabled Corner Pen Shoppe wanted (though years of market survey), or they had trained their customers to want that.
They did not make the same width as Sheaffer, in had they their customers could have made a tragic mistake....and bought a Sheaffer.
Company nib width also depends what ink the company makes or made. Pelikan makes a dry ink so has a wider wetter nib, to bring that to a golden middle. Waterman made a wetter ink, so had a narrower dryer nib....again meeting in the middle.
You are not using a Kaweco ink....so the nib and feed are not adjusted for a Diamine ink. (Kaweco inks are 'new' so I don't know if they used their old formula or a new one, and did or did not match the ink to their feed and nibs.) I would have thought they would have taken a day to see which formula worked or not.
With in tolerance/slop a skinny M can exactly = a fat F....exactly. When you have two different companies, overlap of tip width would be normal. The fatter writing darker.
Just before Sheaffer closed it's US factory, Ron Zorn went to it....and let us into the secret of tolerance.
""""Ron Zorn tolerance
Sheaffer used a dial indicator nib gauge for measuring nib sizes. The nib was inserted into the gauge, and the size read off of the dial. A given size being nibs that fell within a given range. What is listed below were the ranges given on a gauge that I saw in the Sheaffer service center prior to being closed in March 2008.
Measurements are in thousandths of an inch.
XXF = 0.010 - 0.013
XF = 0.013 - 0.018
F = 0.018 - 0.025
M = 0.025 - 0.031
Broad* = 0.031 - 0.050
Stub = 0.038 - 0.050
*there was some overlap on the gauge. May be 0.035 - 0.050.""""
Just because a nib says M means little, because of tolerance. Two different companies have different standards. So part of one's M tolerance could fall into another's F range.
And the 1.0, or what ever the 'new' attempt at a standardization that is offered will also have slop/tolerance......1.1-0.9 would be easily with in tolerance.
The only way to have your nibs all the same width as marked, is to pick just one nibmeister only and have him do all your nibs...........a different nibmeister would come up with a slightly different width.
(I don't see any real reason to do that.....but I'm not as OCD now as when I was 'noobie'.
I grew to know even with in the company, there will be half widths....much less across companies. Is it 'skinny'....good, is it 'middling'..good, is it 'broad'...good.
Take your pick hand grenades or horseshoes is as close as you are going to get with nib widths, unless you are rich and AR.
Then it depends on, flex of the nib, what paper, how hard you press when you write....and some say if the day is humid or not.
So I would think you have a Skinny M and a Fat M....so the fat M is going to put down more ink than the skinny M. =Darker.