Why is current rating for multicore cable lower than single core with the same cross section?
It's important to distinguish between two different concepts:
- Single-core vs. multi-core cables.
- Solid conductors vs. stranded conductors.
A single-core cable contains one electrical conductor, which may be either solid-core or stranded. The picture below shows stranded conductor. Stranded conductor is the norm for all decently-large cables, as solid conductor is very difficult to unroll off the drum and does not withstand flexing or vibration in service.
A multi-core cable contains multiple electrical conductors. The cores are electrically insulated from each other. The cores themselves may be solid-core or stranded. The picture below shows a multicore cable with three cores (two power core and one earth). Again, these cores are stranded.
As a final example, a multicore cable may have many conductors - I have personally installed multicore control cabling with 20 cores (plus earth), similar to that shown below. Typical cross-sectional area is 2.5mm² per core, consisting of 7 strands each 0.67mm diameter. This would be similar to the "43 or more cores" quoted in the table you have linked to.
The table you have linked to refers to ratings for single-core and multi-core cables.
In this case, the de-rating for multi-core cables would be because the insulated cores at the centre of the cable cannot dissipate heat as easily as the insulated cores on the outside of the cable.
The limitation of the amount of current carried by a wire is about its ability to remove heat from the current carrying areas.
In a single strand wire the wire is heated by the current flowing through it. This heat is removed through solid copper to the outside of the wire.
In a central strand of a multi strand wire the heat has to go through the contact area between the strands or the air gaps between the strands to get out of the wire.
A greater effect will apply to a bundled multi core cable. The electrical insulation between the wires will add to the thermal insulation and the ratio of heating to overall surface area will increase hence increasing the temperature.
If you look at the circle packing link suggested by Vladimir Cravero you will find that the ratio between number of cores hence heating effect and circumference of the enclosing circle hence heat dissipation has approximately the derating ratio given in your table.