No, it does not fluctuate. That was some leftover from the beta that was not edited out of the wiki until late.
Pretty sure you are incorrect. Look at code below from the common/terrain file.
forest = {
color = { 89 199 85 }
movement_cost = 1.5
combat_width = -0.20
ai_terrain_importance_factor = 5.0
sound_type = forest
units = {
attack = -0.2
#movement = -0.2
}
enemy_army_bonus_air_superiority_factor = -0.1
}
hills = {
color = { 248 255 153 }
movement_cost = 1.5
combat_width = -0.33
ai_terrain_importance_factor = 3.0
sound_type = desert
units = {
attack = -0.3
#movement = -0.2
}
enemy_army_bonus_air_superiority_factor = -0.05
}
mountain = {
color = { 124 135 125 }
movement_cost = 2.0
attrition = 0.4
combat_width = -0.66
ai_terrain_importance_factor = 10.0
sound_type = desert
units = {
attack = -0.6
#movement = -0.4
}
enemy_army_bonus_air_superiority_factor = -0.10
}
plains = {
color = { 255 129 66 }
movement_cost = 1.0
ai_terrain_importance_factor = 0.1
sound_type = plains
}
urban = {
color = { 155 0 255 }
movement_cost = 1.2
ai_terrain_importance_factor = 4.0
sound_type = plains
units = {
attack = -0.3
#movement = -0.2
}
enemy_army_bonus_air_superiority_factor = -0.50
}
jungle = {
color = { 127 191 0 }
movement_cost = 1.5
attrition = 0.3
combat_width = -0.25
ai_terrain_importance_factor = 6.0
sound_type = forest
units = {
attack = -0.3
#movement = -0.3
}
enemy_army_bonus_air_superiority_factor = -0.25
}
marsh = {
color = { 76 96 35 }
movement_cost = 2.0
attrition = 0.5
combat_width = -0.25
ai_terrain_importance_factor = 8.0
sound_type = forest
units = {
attack = -0.4
#movement = -0.4
}
}
desert = {
color = { 255 127 0 }
movement_cost = 1.05
attrition = 0.2
ai_terrain_importance_factor = 8.0
sound_type = desert
}
The above is straight from the file. Notice the bold modifiers. And I know that last night when i was playing around with tactics to invade Iceland my combat width was down to 60 on a beach invasion into either hills or mountains (can't remember offhand which).