Maybe it is just asynchrony, and geometry takes longer for the brain to see and grasp onto. Did any of your advanced math kids take longer to developmentally grasp onto geometry?That would be my guess.
Kids who are computationally fast may seem to FLY through early math concepts (which, let's face it, don't ACTUALLY get complex until algebra-- until then it's more or less the same five or six ideas done to death procedurally).
But yeah, there is a spatial maturity that
has to come to grasp some concepts in geometry.
Other things that seem to be possible quirks related to asynchronous development:
sequencing
patterns
logic-puzzles and proofs
writing expressions that relate to real-world problems
Six is just really young still, even though I know it doesn't seem that way with one of these kids.

DD is 14, and we
still run into occasional holes that seem more developmental than anything else. It's like she CAN'T actually learn them yet. I don't really get concerned about this sort of thing until she's looped back to it again in six to twelve months with no notable improvement in mastery or progress in learning it.
With that said, though, I
did worry about it PLENTY at 3-7yo. DD simply COULD not extend patterns when she was four. She
could not do it. She could divide fractions. She understood multiplication and negative numbers...
but triangle, square, circle, triangle, square, ? threw her into a tizzy. I can remember her just crying and crying with Reader Rabbit saying "I'm sorry! Try Again!" in the background.
A year later, it was as though it had never happened-- it was just EASY for her.