I may not be an oh-fish-uhl scientist, but I can provide an answer or two.

First, the epicenter is just the point on surface of the earth directly above where the earthquake started. The actual point down in the ground is called the hypocenter.

This is important because maps are by their nature two-dimensional, but reality is different.

Not all faults are vertical. Many dip at an angle. So, when a quake occurs, especially on a shallow dipping fault, the epicenter can be many miles from the surface trace.

You are also partly right that faults are not single "straight lines". Many faults are more accurately described as "fault zones" which may be many tens of meters wide. Note I said meters, not miles. But this does not preclude parallel strands - fault zones in their own right - some miles apart.

Further, most fault maps only show the major and some minor faults. There are actually many more faults that usually don't get mapped. Further, the crust is actually riddled with faults. It may be that some of these tiny quakes occur on short segments of fault strands unknown and unmapped, but certainly related to the major fault that is mapped.

Hope this helps answer some of your questions. I'm sure there's more to it than the above, so all you "real" scientists better jump in.

Brian