Season of birth, soccer, childhood intelligence

First, from the Freakonomics guys:

If you were to examine the birth certificates of every soccer player in next month's World Cup tournament, you would most likely find a noteworthy quirk: elite soccer players are more likely to have been born in the earlier months of the year than in the later months. If you then examined the European national youth teams that feed the World Cup and professional ranks, you would find this quirk to be even more pronounced. On recent English teams, for instance, half of the elite teenage soccer players were born in January, February or March, with the other half spread out over the remaining 9 months. In Germany, 52 elite youth players were born in the first three months of the year, with just 4 players born in the last three.

Next, we have the BPS Digest folks, with another interesting correlation:

Countless studies have found that children’s intelligence appears to be related to the time of year they were born in. ... Reading ability at age 9 and arithmetic ability at age 11 were both related to season of birth (children born in late Winter or Spring performed better) ...

The explanation for the phenomenon is quite interesting. Let's listen to the Freakonomics duo:

Since youth sports are organized by age bracket, teams inevitably have a cutoff birth date. In the European youth soccer leagues, the cutoff date is Dec. 31. So when a coach is assessing two players in the same age bracket, one who happened to have been born in January and the other in December, the player born in January is likely to be bigger, stronger, more mature. Guess which player the coach is more likely to pick? He may be mistaking maturity for ability, but he is making his selection nonetheless. And once chosen, those January-born players are the ones who, year after year, receive the training, the deliberate practice and the feedback — to say nothing of the accompanying self-esteem — that will turn them into elites.

In his excellent Scientific American article on The Expert Mind, Philip Ross concurs:

... [S]uccess builds on success, because each accomplishment can strengthen a child's motivation. A 1999 study of professional soccer players from several countries showed that they were much more likely than the general population to have been born at a time of year that would have dictated their enrollment in youth soccer leagues at ages older than the average. In their early years, these children would have enjoyed a substantial advantage in size and strength when playing soccer with their teammates. Because the larger, more agile children would get more opportunities to handle the ball, they would score more often, and their success at the game would motivate them to become even better.

Okay, that's about expertise in soccer. How about childhood intelligence? Why did Aberdeen children born in winter and spring months do better in reading and arithmetic tests? Here's the BPS Digest again:

... [S]eason of birth was only related to later intelligence because it affected the age children started school, with those who started school younger or older than the average tending to score less well on later intelligence tests.

Hmmm, being older gives you an edge in soccer (and we seem to have an explanation for it). But it seems to be of no help in intelligence tests; what might be the explanation for this?