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.