Are you moving your troops along different terrain? Note the battle_map modifiers. They will be changing a little for 1.1, but they currently are:
Code:
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
climate default
grass_short 0.87
grass_long 0.80
sand 0.70
rock 0.70
forest_dense 0.65
scrub_dense 0.70
swamp 0.60
mud 0.65
mud_road 0.75
stone_road 0.85
water 0.50
ice 0.65
snow 0.70
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
If marching through various terrain in a single march-order then they'll change their speed