I always took it to mean that the average loading of all the wires in the conduit was at about half the total calculated load.
However, now I'm wondering if my reasoning is circular. Maybe it's one of those things that you think you understand, but can't explain it.
If 12 #6 THWN copper conductors (basic ampacity, 65 amps per Table 310.16) are installed in a single conduit, Table 310.15(B)(2)(a) requires a reduction to 50%, or 32.5 amps.
If it can be shown that, at any given time, either half of the conductors would be carrying no current while the others are fully loaded, (or another combination of percentages that would equal the same total current) you are permitted to alter your adjustment factor up from 50% to 70%, for an increase from 32.5 to 45.5 amps.