Very good question!  One quick note: the climatology line in the CPC/IRI probability is based on 1981-2010 only (this has to do with the fact the World Meteorological Organization (WMO) recommends national meteorological services use this as a modern era baseline).  Regardless, keep in mind that La Ninas can last for multiple years while El Ninos do not.  The climatology line is per season and doesn't care whether La Nina has lasted for two years or one.  So you can have more (shorter) El Nino episodes and fewer (longer) La Nina episodes and they balance out for an individual season (i.e. just within NDJ).  Hope that makes.