Maybe, given that William is away with the football, Pippa or Carol will be in attendance to support her tomorrow but couldn't do today.
or she wanted to bring Charlotte with her but Charlotte had a birthday party invitation this afternoon.
or she thought "fuck it if I'm going to get dolled up for the afternoon I want to watch Alcaraz's bum in his shorts not Paolini's"
or Roger was intending to attend tomorrow and she wanted to go with him
or, more likely, none of us will ever know.
i hope she has a lovely time.