ok, that is a major typing error of mine.
the real question should be: why do SCYTHED chariots get a negative bonus when later in 100 ad ( guesstimation) the romans used horses with blades at the side for hamstringing.

the romans didn't use chariots, sorry, they used scythes. probably on cataphracts or somethning they seemed to use later on. maybe it was even later, i have no idea.