Other than belonging to differnt genuses (or is it geni), they have different light and water flow needs. Also they react differently. The Blasto will get along with anyone (peaceful), the Acan might fight (chemically and with sweepers), and the favia is most apt to send out stinging tentavles at night.
Most on-line vendors will list lighting, waterflow, difficulty to keep, and if thery are peaceful, semi-aggressive, or highly aggresive (towards other corals).
Also, the question isn't dumb.
The dumb question would be why is half of the flesh of my Blasto missing? It looked beautiful sitting next to my favia yesterday.