Well, from a business point of view, the market forecasts are generated from pst performance. Therefore it makes perfect sense that the first releases for each platform would be in keeping with the current market segmentation (for each company itmight differ slightly, and I have no idea what the overall percentages are).
This doesn't figure any large incentives made by either camp in the initial marketing putsch of a new hardware generation, though. I wonder how much Sony and M$ are prepared to pork-barrel ...