The "auto" part is for when there is a lot of headlights behind you. I have noticed this at night when there is a car behind and the light is rather bright at first, but the rear-view mirror "auto"dims.
2018 C-HR XLE Ruby Flare Pearl R-Code
2010 Rav4 (200k mi club)
2004 Sea-Doo GTX with custom fishing rig.