I believe this is the main reason for the premium. FNMAS is almost as liquid as FNMA and FMCKJ is just as liquid as FMCC, using 1-year dollar volume totals.
Those who are truly holding prefs for the long haul are in the less liquid names, which are trading at almost historical discounts to the liquid series. I have been swapping down the div ladder since the recent liquid pref spike and intend to swap back up once this trend dies down; this cycle has happened many times in the past.
Market Data 
Markets 
