Hasnt got anything to do with that, I am in the same both (Global Entry/Nexus and GGL). All about how you have your travel patterns (likely what happened to [MENTION=566057]KARFA[/MENTION] above, due to creative routings), checked luggage vs non checked, how far in advance you bought your ticket etc. I once had US immigration telling LHR that I couldn’t board an AIr Canada flight to YYZ (bought same day). When asking why, they called US immigration and was told that flight passes through US airspace so have to be cleared. All this despite Global Entry etc. Almost missed my flight. Managed to get an AC agent to escort me to plane and not close the gates, and this was despite arriving at airport 2 hours in advance.

However, yours seems much simpler, likely only a plain SSSS on your boarding pass