Doesn't make a whole lot of sense, for PS4 and XBO sales to drop in December. Usually December is much stronger than November. If this is true, then.. wtf happened??
Based on last year, I'd say this just about follows the normal trend. Nintendo systems are significantly up, Sony is significantly down,and Microsoft stays close to flat. I think we see this due to Sony putting huge sales on there systems in November, but not in December. Nintendo didn't do many deals, at least ones that you want to jump on. Which leads to people waiting and seeing on the Switch, that leads to a much stronger December for Nintendo and a weaker one for Sony and Microsoft.
November 2017 http://gamrconnect.vgchartz.com/thread.php?id=233441&page=1
PS4 > 1.582m
XBO > 1.376m
NSW - Between 770k and 900k
December 2017 http://gamrconnect.vgchartz.com/thread.php?id=234263&page=1
NSW - 1.5m
XBO - 1365k
PS4 - 1080k