In this case ,we only discuss the "起" and "回" as a measure word .
Second , when will we use 起as a measure word?
For example :一起(个)案件 a law case , 一起事故 a accident
when will we use 回as a measure word?
For example :一回合 one/a round ,一回戏（a/one drama ）
As jemesjin said , 一回事：one kind of things( no deference between A and B)