People did studies on that? That just seems so frivolous!
It isn't an obvious thing. .999....=1 is a problem of understanding how the real numbers work, and actually goes so far as to sometimes impact otherwise good calculus students and the like. Algebraic manipulation is largely formal without regard to the conceptual meaning. While I would have guessed that there would be such a correlation the issue isn't obvious. So no, it isn't frivolous.
I'm mostly asking this open question to those among us who are well-versed in developmental psychology (I'm mostly thinking of children) . Although, failing the actual scientific research on the topic, I guess some testable hypotheses would be great too.