The cores conduct light in fiber bundles, but they are surrounded by cladding layers.
Bundles are made with thin cladding layers, but some light must fall onto the cladding rather than the core. The fate of that light depends on the bundle design. Fibers in rigid bundles have a dark outer cladding layer that absorbs light so that little can pass between fiber cores. Light that leaks out of the cores of individual fibers in flexible bundles cannot easily enter other fibers. However, neither type can completely prevent light from leaking between fibers.
Most light entering the cladding is lost, which can limit transmission efficiency. This makes the fraction of the surface made up by fiber cores an important factor in a bundle’s light-collection efficiency. That is, the collection efficiency depends (in part) on the packing fraction, defined as

Packing Fraction = total core area / total surface area

A typical value is around 90% for lighting fiber bundles. And 60~80% for imaging fiber bundles.

Note: Light that falls into fiber claddings in bundles will have light lost.