time and capacity

Malcolm Gladwell writes in his new book:

. . . excellence at a complex task requires a critical, minimum level of practice – which surfaces again and again in studies of expertise. In fact, researchers have settled on what they believe is a magic number for true expertise: 10,000 hours.

Does anyone else find it suspicious that the magic number does not depend on the field? Perhaps it does not measure the amount of study necessary for expertise, whatever that is, but a point beyond which improvement is much more difficult because a brain’s capacity is finite, which then becomes our definition of expertise.