What time frame was the data compiled?

Last 20, 30, 40 years?

http://www.espnfc.us/ajax-amsterdam/story/2313733/ajax-train-most-active-players-in-european-top-divisionsstudy-says

Ajax have trained more footballers currently playing in top-division European leagues than any other club, according to a study by the International Centre for Sports Studies.

CIES presented a map of the top 100 clubs who have trained the most players in 31 European leagues, and Ajax came out on top, producing 77 players.

The Dutch giants narrowly edged Serbia's Partizan Belgrade, who trained 74 players. Barcelona were well behind in third place with 57 players.

France's Ligue 1 had the most clubs in the top 100.

The data tracked teams where players trained for at least three seasons between the ages of 15 and 21.

CIES study: Most active players trained, top 5 leagues

3. Barcelona (ESP) 57
7. Real Madrid (ESP) 47
15. Lyon (FRA) 40
18. Manchester Utd (ENG) 39
22. PSG (FRA) 35
28. Arsenal (ENG) 33
31. Bayern (GER) 32
33. Monaco (FRA) 30
40. Lens (FRA) 28
44. Rennes (FRA) 27
44. Stuttgart (GER) 27
49. Athletic Bilbao (ESP) 26
49. Real Sociedad (ESP) 26
49. Valencia (ESP) 26
49. Metz (FRA) 26
49. Montpellier (FRA) 26
59. Bordeaux (FRA) 25
66. Atlético Madrid (ESP) 24
71. Manchester City (ENG) 23
71. Internazionale (ITA) 23
82. Caen (FRA) 22
82. Toulouse (FRA) 22
82. Atalanta (ITA) 22
92. Sevilla (ESP) 21
92. Nantes (FRA) 21
92. Lille (FRA) 21

What do they do well ? And why can't every a league club replicate it