My DD was like that and did jigsaw puzzles reversed where the only thing she could see was the solid color of the shape beneath. I didn't really think about it, she just started doing it because she was bored of doing the picture side.
It's probably just packaged into the whole visual-spatial skill strengths.
I didn't think to much of this either but other people used to remark about it, that from the time she was identifying letters, words and started reading it didn't matter what type of font it was or whether is was forward/backward/upside down. I found out that there is a little educational science to the whole font thing with learning to read but DD was well past that by then.
It's just part of their package.
