Difference is PS4 only had 1 and a half months in 2013, while Switch had 10 full months in 2017.
This way makes the most sense because it's the closest you can get.
2016 started with PS4's 777th day on the market, 2019 began with Switch's 669th day on the market. A gap of 158 days.
2015 v 2019 would be a gap of 257 days.
I'm not constesting which should be the comparison base years.
I'm saying 2016 was PS4's 4th year or PS4's 3rd full year.
Meanwhile 2019 is Switch's 3rd year or Switch's 2nd full year.
He can compare anything he feels like making a comparison.
But they are not the same 3rd year as the OP says. That's all.