It seems to parallel with summer season to Europe. Many upgrades released shortly before the flight. With many short-notice trips I am often on the WL for transatlantic flights, but ALWAYS *) cleared (1K).
Always referring to over 10 years.

*) Safe 1 screw-up by an IAD gate agent 2 years ago, who had my seat, took my cert and then goes: "Oops, your seat is gone!". To this very day I have no idea, what in the world happened there.