Actually. They may contain a mix of more than 10 teams, but all with the same starting line up. I can't really see the point of starting the same 10 teams in each league as it wouldn't be possible to get different results in different leagues.
Good point, and definitely a possibilty.
Now that you say that, more likely than my scenario.
There are "Trading Leagues" that do that.
Everybody starts with the same team, and the best trader wins.
If they had 6 Leagues it would make even more sense. 15 teams each appearing in 4 Leagues, or 20 teams each appearing in 3.
5 x 10 team Leagues doesn't lend itself to an even distribution, unless it is 50 teams appearing in one League each!