EEE-INST-002 Wire Derating - Single Wire vs. Bundle/Multi-Conductor
Imho the 25A is for a single instance of the cable in an open situation . As soon as you group 4 well 8 together into a box the cooling is reduced so the current capacity has to be reduced.

This explanation is the same as cables laid open in a loft or attic or wall with an air space. But as soon as the cables are covered or surrounded by insulation the current capacity is reduced.