The rule does not say decathlon is only ten games. It says players may play in all decathlon games but only the first ten games they play in count towards decathlon. "All Eliters Members will be eligible to participate in all Decathlon Tournaments. However, will only be accumulating points for the first 10 Decathlon Tournaments."
Each league decides what is best for them and hosts decathlon accordingly. Some host 10, some 15, and some 20. By hosting 15 or 20 decathlon tourneys, it allows players who work or have other things going on during some of the tourneys to still participate in 10 games.
For more information regarding Decathlon see Help Topic 2533 - League Decathlon.
I hope this answers your questions.