The uncanny is something that is usually more mysterious and disturbing, rather than surprising. I find an uncanny likeness to be a similarity that is strange and not complete. This is why twins can give an uncanny effect.
The 'Uncanny Valley' posited by Masahiro Mori in 1970, is a diagram that I think explains the uncanny very well. The more realistic a cartoon becomes, the more you notice the gap between it being real. Cubo girl looks very realistic, but doesn't look it when moving, giving an uncanny effect. You don't tend to notice this with cartoons such as the Simpsons, as it is not meant to look realistic, so it is not seen as strange.