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Consider this graph of engine thrust versus specific impulse (from https://dawn.jpl.nasa.gov/mission/ion_prop.html):
Most propulsion technologies encompass roughly rectangular regions on the graph. Electric propulsion is the union of two rectangular regions (ion and magnetic). However, chemical propulsion has an irregular shape.
Why does chemical propulsion have this irregular shape?
I'm guessing that the chemical rocket envelope in the plot encompasses points representing actually-built rocket engines, rather than theoretical ones, hence some of the irregularity of the shape is due to historical accident.
10N is quite small for a chemical rocket engine. Such units are mainly used for attitude control of small spacecraft rather than making significant maneuvers, so reliability, simplicity, and light weight are more critical than specific impulse. I'm not sure if it's not actually possible to make high-Isp small thrusters, or if it's just that no one bothers.
In particular, I note that Aerojet's catalog of bipropellant (MMH+NTO) thrusters (which get around 300 seconds Isp) extends down to 22N; below that, they offer catalyzed hydrazine monoprop thrusters (around 220 seconds) down to 1N; the engineering simplicity of requiring only a single propellant tank pays for the loss in specific impulse (and it's probably tricky to get good bipropellant mixing in such a small combustion chamber). These two categories of thruster contribute to the left two-thirds of the chemical engine envelope in the plot.
Further up and to the right, small hydrogen-oxygen engines stake out the high end of the Isp envelope: the Chinese YF-73 at 44kN and 420 seconds, then a bunch of engines in the neighborhood of the US RL10: 65-100 kN and 440-460 seconds. That's the high-water specific impulse mark for production chemical engines, the space shuttle main engine RS-25 is off the right hand side of the chart at 2200kN and 452 seconds. Again, I'm not sure whether it's possible to make high-Isp hydrogen-oxygen engines smaller than ~40kN or whether it's just not done.
The historical NASA document "SPACE HANDBOOK: ASTRONAUTICS AND ITS APPLICATIONS" has a useful table, which I will partially reproduce here:
TABLE 1.-Specific impulse of some typical chemical propellants
Low-energy monopropellants________________________ 160 to 190.
High-energy monopropellants: Nitromethane_______________________________ 190 to 230
Bipropellants (liquid): Low-energy bipropellants___________________________ 200 to 230.
Medium-energy bipropellants________________________ 230 to 260.
High-energy bipropellants___________________________ 250 to 270.
Very high-energy bipropellants_______________________ 270 to 330.
Super high-energy bipropellants_______________________ 300 to 385.
Boron metal components and oxidant____________________ 200 to 250.
Lithium metal components and oxidant___________________ 200 to 250.
It seems very likely to me that the "Chemical" category encompasses several different fuels, and probably both solid and liquid fuels, which is why it's not a uniform shape.
Someone more expert might be able to guess which specific fuels compose the various areas.
The "Chemical" shape exists because of two clear regimes. This is a bit by chance because of the propellants involved but basically its this:
As an aside the diagram doesn't really clarify the whole range of posiblities for electrically augmented hydrazine monopropellant, e.g. - power augmented catalytic hydrazine thruster (Isp = 250 - 280) - hydrazine arcjet (Isp 550 - 600)
