Lorne - in my view there is no magic to this.
In the past in the UK (main mog market I guess) there has been higher demand than supply.
Mogs don't get retired to a scrappy after 5 years like most Euroboxes do, so the numbers available in the UK are constantly rising.
similarly (I would guess) that the available group of potential purchasers keeps falling, a function of age, demographics, and viable alternatives.
Perhaps the US will buck this trend - if theres enough enthusiasm for the marque and a low enough supply values will stay high for the moment.