If you factor the NPD numbers to the rest of Vgchartz figures for Japan and Others then the 360 has an attach rate of 7.74.

If you take the NPD attach ratio of 8.9 and multiply it by consoles sold in NA then you see that Vgchartz has over tracked 360 software sales in NA by 47,599,137  units now when you take those numbers off of Vgchartz worldwide numbers and divide by total consoles sold you get the new attach ratio of 7.74 worldwide. 

Of course to avoid bias you must assume that if the NPD 360 software attach ration is accurate than Vgchartz is also considerably over tracking the PS3 in the same region as they have the PS3 with 9.1 attach ratio in NA. 

X360 software year ago

PS3 software last week

and actually PS3 launched in Europe one and half year later so in reality the gap is even bigger if you align launches.