Let us look at the numbers again:
http://store.steampowered.com/hwsurvey/videocard/
ATI Radeon HD 5800 Series
-3.21% from last month.
ATI Radeon HD 5900 Series
+0.27% from last month.
NVIDIA GeForce GTX 470
+0.64% from last month.
NVIDIA GeForce GTX 480
+0.44% from last month.
And we are talking about 3.29%...which is the number of DX11 systems.
53.18% is the number of DX10 systems.
Now look at these numbers, from the exact same DX11 category your % drops in ATi series came:
ATI Radeon HD 5700 Series 43.46%
ATI Radeon HD 5800 Series 41.85%
ATI Radeon HD 5900 Series 3.21%
NVIDIA GeForce GTX 470 0.65%
NVIDIA GeForce GTX 480 0.44%
So it quite easy to understand the % drops in DX 11 category - After all ATi had 100% in March and now only has like 98.91% of DX11 GPUs. There are THREE TIMES MORE of those elusive 5970 than 470+480! I expect to see even more ATI drops in the next months!
But now lets look at overall picture (all GPUs):
DirectX 11 GPUs 5.58% +1.29%
DirectX 10 GPUs 74.47% +0.23%
ATI Radeon HD 5700 Series 2.43% +0.52%
ATI Radeon HD 5800 Series 2.34% +0.41%
ATI Radeon HD 4800 Series 8.30% -0.13%
As you can see the 5700+5800 got half of the 4850+4870+4890 in six months.
Now lets look at the overall share of the last gen competition to the 5850+5870 (it is a bit hard to compare the 5750/5770, because the first is more a GTS250/9800GTX competitor, while the 5770 is more a GTX260 competitor).
NVIDIA GeForce GTX 260 3.94% +0.10%
NVIDIA GeForce GTX 275 1.22% +0.01%
NVIDIA GeForce GTX 285 1.17% +0.04%
NVIDIA GeForce GTX 280 0.86% 0.00%
It seems the 5700, the 5800 and the 5900 series have been enjoying a nice time with no competition to sell really well.