Side mirror in a car question?

So yesterday I looked out the window towards the side mirror, I saw the the bus sign "not in service", i looked away and looked back and it became inverted and the letters were backwards (as you would expect), any idea why I saw the "not in service" normally in the first place?
Update: could it be the convex surface on the side of the mirror? like the area used for blindspots which inverted it?
Update 2: also.. it was an electric sign, and it does seem logical that i saw a reflection of a reflection, iirc there was a car behind me then there wasn't.
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