I've been thinking alot about this.My theory is it all has to do with where the strain originates.For example sativa tall and lanky...most likely started in a hot humid environment.Indica...short and squat,big fat buds can grow in a dryer environment without growing mold and distroying the flowers.Also theres flowering time-sativa long light cycle,most likely originated by the equater.Indica short light cycle,originates further north or south.
Remember this is a theory and I don't know if it's true.
Sounds right to me.