This isn't a silly or obvious question at all. The key is the concept of the exiting nerve root vs the traversing nerve root. If you have access to Gray's Anatomy 39th edition this is explained and illustrated very well (page 758).
Failing that have a look at this article:
Lumbar and Cervical Radiculopathy
Lets take your example of a disc herniation at L3/L4. the exiting nerve root is the L3 nerve root. It exits the spinal canal just cranially and laterally to the L3/L4 disc so often escapes trespass. The traversing nerve root is the next one down, in this case the L4 nerve root and it passes immediately posterior to the the the L3/4 intervertebral disc where it has little room to move in a prolapse, and it is the one that is most often impinged. So in the lumbar spine it usually (but not always) is the traversing nerve root which exits in the next vertebrae caudal to the intervertebral disc and is in the direct firing line of a posterolateral prolapse