I think we can only speculate on what parameters they consider.
But my thought process is something like this, we know OR have reacted to altnets rolling up in areas which have high OR take up, which suggests holding on to existing customers is some kind of priority for them, as the altnet customers count starts at 0, so nothing for OR to take from them, instead everything to lose.
There is 2 local exchanges to me, that have been skipped by OR, both have really high VM take up, one of them is also a metro exchange (mine). A OR engineer told me at the time of my fault which led me to move to VM, there was less than 8 customers on the cabinet.
Of course my theory is speculation as well, but throwing it out there.
