passengers book seats, not departures.
UA's entire system is heavily 50 seat RJs while most of DL's growth has come with large RJs thus their departures are high relative to seats offered.
even with their use of large RJs, DL has the highest average aircraft size at LAX among the legacy carriers. AA has 119 seats/dept compared to 139 for DL.
AA/US also has an all mainline operation at SFO but DL has some large RJs and still has more flights.
remember also that DL originally loaded the new AUS-LAX flight with the 717 which was rotated from DTW, a 717 pilot base but apparently the operational departments are not ready to spread a fleet of about 40 aircraft over the entire US at this point. by next year, though, the chances are very high that the 717 will be on the west coast replacing a number of the departures being flown by large RJs today (DL has no small RJs from either SEA or LAX).
Since AA is also moving CR9s onto the west coast and will be upgrading some of the flights that operate with smaller aircraft today as well as add a few frequencies, the trend is still that AA and DL are growing on the west with DL doing it from both LAX and SEA while AA is doing it predominantly from LAX.
The increased capacity from AA and DL is bound to hurt other carriers and tip the balance of power on the west coast.