Don't know what temps the bars represent but I do have some insight into this issue.
At freeway speeds, (actually, about anything over ~45mph) my bike keeps a temp differential of about 90 degrees F. That is, the indicated temperature is about 90 degrees above the ambient air temperature. On all my Hondas, it was similar but they had a thermostat set at 180 degrees. So, up to 90 degrees ambient or so I'd see 180 degrees displayed for engine temperature. Above that the engine temp tracked the ambient, just 90 degrees higher. As in, air temp, 100, engine temp 190, air 110, engine 200, etc.
Kawasaki, on this engine design, chose a very low (IMHO) thermostat temperature of 140 degrees. I'm a gen behind with a 2014 but I'll bet it's the same for yours except I have an engine temp gauge, not bars. The response is very similar except it starts much lower. Anything below 50 degrees it sits at about 140. Anything above and the ambient temp tracking begins. Since it does start at such a low temperature the gauge temperature starts to rise at anything above 50 degrees ambient so it's easy to see it going up another bar with temps in the 70s or 80s. What temperature each bar represents is entirely dependent on whatever kawasaki engineers chose them to represent.
And if it was the same engineers that allocated bars on the gas gauge, "linear" has nothing to do with their thought process.
The important thing is to note, as you have, how the bike is responding now so that in the future if you notice it seems to be a bar higher than usual in warm temps or in cool temps it is a bar lower, you probably have an issue with the cooling system.