Cels Forum

(A TV serie need to be composed, at least in Japan,
 of 26 episodes of around 28mins).

Not necessarily.  Recently, because the Japanese economy's been doing poorly, a lot of places have been making short, 13 episode TV series that might in the past have appeared only as OAVs (Lain and El Hazard Alternative World, for example).  It's cheaper in general for them to produce and market television shows than it is OAV series, so you get half-season length shows.