Well, there are 703 "Best" academies in the game. So its really not the "best" academy its the "normal for top managers" academy.
There are only about 100 LA12s and 13s in the game, I don't know how many LA11s - say 300?. So you can see it is very lucky to produce better than LA10. The game would be silly if every best academy was guaranteed one >LA10 per season.
For me, I have had my academy for 4 seasons now, I've produced a handful of LA9s but so far only one LA10 and nothing better even though I always buy extra youths. So far this season, I have a LA9,LA8 and LA7 and I've fired 5 others from the academy, I await friday in anticipation for I receive another 2 reports - maybe I'll get lucky this time