In reference to Table B310-11, what exactly does a 50% load diversity mean?

The factors in the second column of Table 310.15(B)(2)(a) are based on no diversity, meaning that all conductors in the raceway or cable are loaded to their maximum rated load. For load diversity, the user is directed to Annex B.

IMHO,

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.

For example,

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.