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FYAYFD: The reason for lower number could be many, like
- Underestimating the capabilities of the opponent
- The Kingdom isn't a Militaristic or Absolute Monarchy or Dictatorship, so the power of the whole Kingdom isn't centralized. Meaning they can't gather enough troops easily (because each Noble in the Kingdom wants to keep sufficient armed forces with them).
- Lower birthrate in general and slow training of potential military candidates could also be possible reasons.
Even if the society in the story is sort of like a "high middle ages cosmopolitan", but we don't really have the idea of the exact population level, survival rate or the birthrate. And we don't really know what is the exact "healthy farmers to urban dwellers ratio" of that other Kingdom which is going to attack MC's region (we only have a general idea of that for the Kingdom MC is affiliated with), since each Kingdom would have its different ratio and rates, even if they are from the same period of age.