Why is it that most animes have 26 episodes per season? (or 13, which is 26/2 for those of you not so good with math

)
Out of the ones I've seen (still not very many and I know there's many more out there)
Trigun, Cowboy Bebop, Ghost in the Shell (1st and 2nd Gig), Outlaw Star, Gungrave, and Samurai Champloo all have 26 episodes
Hellsing and K-On! both have 13 episodes.
The only one that doesn't fit out of the ones I've seen is Death Note with 37 episodes