Because the committees are just recommending YES/NO based on their specific area of (sometimes "so-called") expertise.
For instance, a committee specialised on power and industry might be totally for opening another coal mine. A committee specialised on environmental issues is probably gonna be against it.
Since ACTA is a trade agreement, the Trade committee is the main committee, and their recommendation thus -- normally at least -- carries the most weight.
In the end though, the representatives in the parliament are the ones deciding whether or not to follow the recommendations of the committees. The EU parliament can be quite unpredictable from time to time...