I copied this off a post in the other forum, which, in turn was copied from an FT forum:
"Originally Posted by UA Insider ![[IMG]](http://images.ibsrv.net/ibsrv/res/src:www.flyertalk.com/get/forum/images/buttons/viewpost.gif)
Hi Everyone, in true FlyerTalk form, your detailed thought and research on this topic are worth talking about. And as usual, we have a pretty boring explanation for what really happened.
As part of combining our two loyalty programs, we did a side-by-side comparison of the mileage calculations used by Continental and United. Of course, we found differences. Most commonly, these differences in calculations were found in locations where airports have physically moved (e.g. DEN, HKG, SIN, etc.). Other differences were found in markets where, when Continental or United added service, the mileage amount chosen was simply set to match other carriers already servicing the market as opposed to doing a new calculation.
In an effort to begin using a single source for all mileage calculations, we refreshed all of our calculation data. The source against which this new mileage is calculates is a standard Great Circle Map (GCM) table. As many of you know, there are several sources for GCM data, and, while they don’t match perfectly, they are also rarely ever off by more than a mile.
Shannon Kelly
Director, Customer Insights
United Airlines"
It doesn't address why the vast majority of changes so far uncovered and reported are decreases. It is possible that there is a more even distribution of increases and decreases, but people will tend to report the decreases....