It's altogether possible that the PC market actually did better in sales than the year before (thus growing their own percentage), but dropped in the overall market because of far greater growth in the console markets.
It is also irrelevant. The drop is what matters. If PC game sales increased versus the prior year, it just made the drop in overall marketshare a little less severe. Console games are becoming more dominant, and it is hurting the development and release of new PC games. I personally don't want to see it continue, but as the next-gen consoles start to deliver more of what PC gaming offers, I fear the trend will accelerate.
This comment was edited on May 25, 15:20.