In addition to the game difficulty, there's a hidden game stat that make enemies more powerful. This stat is calculate so it's up to the current player level: it's calculated depending on the player elapsed time game days stat. This mean that the more the player has played, the more enemies will get stronger. Now, before we get on how this coefficient's calculated, here's what this coefficient changes: it simply multiplies any enemies damage by its value.

Here's the code extract of where this coefficient's calculated:

# Calculate the enemies global damage
# coefficient, depending on the player
# elapsed time in game-days
#
# Here's the calculation settings:
# x < 25days  => .85
# x < 45days  => .95
# x < 50days  => 1
# x < 80days  => 1.15
# x < 100days => 1.25
# x < 150days => 1.35
# x < 220days => 1.45
# x < 300days => 1.5
global enemies_damage_coefficient
enemies_damage_coefficient = 1  # placeholder
if player["elapsed time game days"] < 25:
    enemies_damage_coefficient = .85
elif player["elapsed time game days"] < 45:
    enemies_damage_coefficient = .95
elif player["elapsed time game days"] < 50:
    enemies_damage_coefficient = 1
elif player["elapsed time game days"] < 80:
    enemies_damage_coefficient = 1.15
elif player["elapsed time game days"] < 100:
    enemies_damage_coefficient = 1.25
elif player["elapsed time game days"] < 150:
    enemies_damage_coefficient = 1.35
elif player["elapsed time game days"] < 220:
    enemies_damage_coefficient = 1.45
elif player["elapsed time game days"] < 300:
    enemies_damage_coefficient = 1.5
else:
    enemies_damage_coefficient = 1.5

This means that:

• Between 0 and 25 game days, the coefficient is equal to .85
• Between 25 and 45 game days, the coefficient is equal to .95
• Between 45 and 50 game days, the coefficient is equal to 1
• Between 50 and 80 game days, the coefficient is equal to 1.15
• Between 80 and 100 game days, the coefficient is equal to 1.25
• Between 100 and 150 game days, the coefficient is equal to 1.35
• Between 150 and 220 game days, the coefficient is equal to 1.45
• Between 220 and 300 game days, the coefficient is equal to 1.5

Here's a great graph that represents the coefficient (Note that the player chosen difficulty also affects this stat):