It means that when originating from LHR, AA is willing to open up a lower fare bucket (e.g. D/I) than for those originating from JFK, based on various factors, including total ticket value, competitive fares/availability on the LON-RIO market, etc. etc.

Even though it probably doesn't make much sense, C availability seems to be implemented by linking it to those lower-fare buckets, so you get the effect that you see.

This is not an EF issue, it's the way AA works.