So I suspect that my experience can then be explained due to the large number of *SE folks on this route looking to upgrade. So in the end it ends up being fare class within that pool. Since I'm only allowed to purchase the least expensive tango plus fares that pretty much places me near the bottom unless there are other factors.

Andrew, when you say 'status' is that a purely binary thing or does that have shades of grey - eg # status miles this year/last year etc?

Similarly with class of fare you can purchase the same classs of fare but at different prices, do these factor into the sort for upgrades?