An image containing the back of the sign doesn't contain a sign. A sign is the front. The back is just a flat surface, not a sign.
But it's part of a sign and all things that are part of a sign, as long as thry are attached to the sign, are signs. ;)
I have the same problem.
To pass Google CAPTCHA you have to perform like an average human. This is different from getting it right. Once I started being lazier, (e.g. back of a sign isn't a sign, picture that contains a storefront but in the distance doesn't contain a storefront), I have had greater success rates.
It's like Google is training me to be dumber.