This is fascinating. So it looks to me like the proposals is that our brain has a map from 6 -> "multiples of 6" and 8 -> "multiples of 8" and it triggers both nodes, and that our brain then finds the MOST stimulated node (i.e. numbers that are multiples of both 6 and 8).

Because 24 is a multiple fo 6 and 8 it's causing interference.

This seems believable, curious if there's hard data on this (that indeed 6x8 is the hardest and other numbers with shared multiples) but not curious enough to look it up.