I have analyzed snow density is some more detail here (scroll to bottom of page), using daily data to compare not only average density but distribution of snow densities at Alta, Mammoth and Mt. Mansfield: http://bestsnow.net/snoqlnet.htm .
The avalanche research community classifies snowfall locations as Coastal, Intermountain or Continental. The Wasatch, Tetons and Selkirks/Monashees of interior B.C. fall in the intermountain sweet spot of high quantity and fairly low average density of 8-9%. Japan's midwinter snow also tends to average in that 8% range, because it's mostly "lake effect," which as a poster above noted, tends to be much drier than one would otherwise expect. The lowest average densities are in the Continental climates of Colorado and Alberta, but quantity tends to be lower and avalanche risk the highest. Thus there's a lot of skiing on the subsurface because there's not enough new snow at one time for flotation. That said I've had 5 ski days lifetime at Castle Mt., 3 of them were powder days and one of them was a top 3 powder day lifetime for lift service.
I have extensive ski experience in both Utah http://bestsnow.net/vft_utah.htm and interior B.C. http://bestsnow.net/vft_croc.htm . The comment about better midwinter snow preservation due to less and weaker sun in B.C. is true. Unfortunately the comment about winter rain incidence due to lower altitude is also true. I had 3 days of cat skiing immediately after the 2005 Tropical Punch and they were some of the worst snow conditions I've ever skied. Obviously that's the exception because I'm still going to B.C. for cat skiing nearly every year, including 8 days scheduled for January 2012 at Mustang and Baldface. With regard to Utah I own a timeshare week at Snowbird, which I still regard as the best lift served skiing in the world with 120+ lift served areas skied, and always looking to add a few more each season.