I would imagine that “zero” sunspots back in the 1800’s and 1700’ss and even perhaps the 1990’s did not detect all of the sunspots if they were small enough. So - a “zero” in 1800 might mean there were still 5 sunspots, so if we just see one, that might be a -4 using the 1800 scale!?
That’s sorta where I’m going, but I really don’t know much about the Science of the Sun.
I think that Sunspots have to do at least with the energy of the surface of the sun.
With the progress of remote Sun sensing in the last half Century, I’d guess that there are likely other observable characteristics that could provide measurements of Sun Energy below the level of Zer0 Sunspots, and thus extend the scale.