We know the following from this and previous threads:
... there has to be a metric other than total spend and FFM and the only metric that remains to distinguish between such travelers is yield (cents per mile)...
Except we know that neither FFM nor revenue is entirely determinative of invitations because of the discrepancies we have seen time and time again between those invited and those not.
Agree with your premise that there may be other considerations than the raw measure of "how many miles of F/A/C/J etc. did you fly?" (at least this year; in the preceding 3 years, requalifying was a simple matter of meeting the required number). But your statement that "the only metric that remains to distinguish between such travelers is yield" assumes United knows nothing else about its customers - and that simply isn't true. In many cases, they know who you work for, and that could positively help FF's who work for a big customer; another factor they certainly know about is whether you are a complainer or not; and they know where you fly to and from. Yield is not the only thing that United cares about; they're interested in rewarding influencers, and they're interested in keeping their competition at bay.