Hi mate they do this as not all bolt holes on all motors will line up on all arms so they give you 2 bolts ( its cheaper for them ) but if you look at spec of arms it may say 16mm and 19mm bolt holes so you can fit a motor thats all 16mm holes or 19mm if you look at them they are off set so you have 2 holes 16mm 2 holes 19mm apart if you can get all 4 to line up then use 4 bolts , its a cheap company that only gives you 2 bolts per motor