Let's say there's a kid who plays tennis for hours every day. Tennis is how he plans on making a living, and he already makes a little money at it. He hits a great one-handed backhand. When he's in school, he has a one-hour tennis class every day where the coach makes him hit a two-handed backhand because that's what they teach at school. You test the kid and wouldn't you know it, he's a lot more consistent with the one-handed backhand than with the two-handed backhand. Now, would you take this as evidence that there are any particular difficulties in teaching a two-handed backhand as compared to a one-handed one? Maybe you would conclude that the two-handed backhand is intrinsically more difficult to learn because the reduced reach requires better court positioning and footwork, which are less concretely related to the task of hitting the ball over the net.
Or maybe you just conclude that the kid practices one skill for hours every day and the other skill briefly and sporadically.
There's really no need to construct a reason based in armchair cognitive psychology to explain why kids are better at a method they practice a hundred times a day than at methods they might practice a few hundred times a year, assuming they go to school regularly and do math problems every day while they're there.