Maybe a trivial question. My iPhone SE has 4 bars to show the signal strength. I was in the AT&T store the other day looking at the iPhone models on display, and they all showed signal strength as 5 circles. The rep came over and couldn’t account for what determines which is used. He took out his phone and it also showed bars. I’m guessing it’s IOS version related, as the display phones were on IOS 10 or early 11. The rep and I have the latest version.