Using % for all the year is a bad calculation. Just because in those months those were the % doesn't mean when holiday season arrives PS4 will sell 20% less and Switch 30% more than in 2018. It doesn't work that way.
PS4 was selling less during the year last year but when holiday season came, the % difference was more closer. As for Switch, Switch was behind during Jan-Oct 2018 compared to Launch-Oct 2017 and thanks to a way better holiday season it sold 3M more.
I'm pretty sure Switch won't have a 30% better Q3 in 2019 than in 2018, and considering almost half of the sales from Nintendo consoles usually come during that period, you should expect a lot less than 20'9M sold...In my humble opinion, Switch will aim this year to reach 18M sold to consumers, and right now it's on pace of doing it, but we still need more data and time to know what to expect for all the year.
And because holiday season 2019 for PS4 can be similar to 2018, you can expect PS4 being more than 15M too.
That's what i think will happen.
I know, I was giving a general idea of how sales are trending so far, of course they aren't going to be exactly that.
I think you're mixed up about PS4 last year.
Jan-Oct 2017, 13.02m
Nov+Dec 2017, 7.11m
Jan-Oct 2018, 11.71m (-10.1%)
Nov+Dec 2018, 6.29m (-11.6%)
The holidays were down more than non-holiday months.
There is nothing announced for April-October that will change it's current trend as last year had God of War, Spider Man & Red Dead and this year doesnt currently have anything close to their sales potential. There is also zero reason to believe this holiday wont be comfortably down YoY for PS4.
As for Switch I have a hard time believing it will only be up ~10% YoY for the full year when its already up ~28% with the last big title having released 2 months ago. Its lineup from June onward is more stacked than last year and it is heavily rumored to be getting 2 revisions later this year.
It might not be up ~28% for the whole year but anything under 20% would surprise me.
When the herd loses its way, the shepard must kill the bull that leads them astray.