Is that really notable? They are both small numbers, and those NT machines are likely at companies with upgrade policies versus people who choose not to upgrade their personal macs for free.
Also there are many valid reasons to not upgrade from NT...
Not if you expose your machine to the internet!
I suspect NT wins unless there are unpatched TCP/IP exploits.
I wonder what percentage of in-use ATMs make a similar bet.
There are some nt machines chugging along doing specific tasks that will outlive us all.