Looking at ExpertFlyer for C availability, I have run into cases where if I search just for a nonstop I get C=0 but if I search from a different origin that can connect then on the originally displayed segment I can see C>0.

For example, if I search ORD-LHR on 12/25, AA86 shows C:0. However if I search PHL-LHR on the same day I get AA 1994 connecting to AA86 with C:2 on the latter (the former of course has A not C).

Does the flight in this example have any C availability or not? Why would the two searches show different results? As far as I know, C upgrades do not have married segment logic, since the segments can upgrade independently.

(Note that I am not trying to book a specific flight at this time. I just trying to get a feel for what routes might let my SWU clear vs. routes where I should just pay for business class outright.)