This is natural as numbers get higher. For example, let's say Sony sells were 50,000 per week. To sell double 360 or Wii only have to sell 100,000. But now we are talking about Sony sales of 160,000 per week. to double 360 and Wii have to do 320,000. In the case of the Wii they are exceeding supply to get there so they simply can't reach it. In the case of 360 they are running out of certain models (Elite was hard to find for a few weeks), and to sell 160,000 more is a lot different then only having to sell 50,000 more. Market share differences are a creative way of looking at it.
But, the actual console gap, (# of consoles sold) against each other is increasing... 360 was ahead of PS3 by 50,000 a week for a while. Now it is 90,000. So, the number # of actual consoles in the gap between them is getting larger, not smaller as perceived by showing it as market share changes. Getting a bigger piece of the pie doesn't help, they have to exceed the gap before they truely start to gain ground.