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.
I see this rather in favor for Mac OS X as their upgrade system definitely works, 10.8 is not old at all (2012), Windows NT is from the 90s mostly pre-internet. It's putting things into perspective.
I wonder about NT with nothing but up to date ssh exposed vs win 10 with nothing exposed except the pile of obscure phone home protocols you can't turn off.
I suspect NT wins unless there are unpatched TCP/IP exploits.
I wonder what percentage of in-use ATMs make a similar bet.