I'm not trained in this stuff, but the primary voltage numerically will depend on where its measured.
On the positive side of the coil is going to be system voltage when the points are open. It will drop when the points close and current flows through the resistor. The resistance to flow causes the voltage drop.
On the negative side of the coil, there is both the resistance of the ballast and the coil itself. It may not be ground votlage because of the condensor in a points system. With a transistorized system it may be ground.
My understanding is that the voltage is also effected by the inductance in the coil, and the fact that the switching is not perfect. With points we use a capacitor (condensor) so see the effect of that as well.

On this page there is a nice illustration of coil current superimposed on coil voltage.
https://www.sw-em.com/Ignition_Additional.htm

Notice the voltage steps while the current ramps and then becomes stable.
I beleive that's the current that makes the tachometer needle move. The more frequently it occurs, the more the needle moves.


There's more along these lines here
https://www.picoauto.com/library/automotive-guided-tests/primary-voltage-vs-current


edit: here's a more detailed explanation that I found helpful. The tranformer does cause the primary side to somewhat reflect the secondary voltage changes.
https://www.motor.com/wp-content/uploads/FireInTheHoleUnderstandingIgnitionWaveforms-May2005.pdf

I had Faria tachs on most of my cars back in the 60's. All the voltage for the coil passed through the tachometer. There was no separate signal feed to the tach like is shown on this diagram.