If you see the footage he arrives at the door first, having walked with Fergie and Tim, but has to wait for Anne - who was behind chatting with Zara and Mike, to go in first.
He also wasn’t sat next to Camilla either. The working royals - Anne (plus Tim) and the Gloucesters were sat next to her before him. Princess Alexandra (with her daughter for support) who has never official retired from royal duties was also in that row, so working royals were taking precedence over order of succession.
had William been in attendance I think he’d have been in the row behind, but he was the easiest to shuffle around.