Why is everyone surprised at Harry's Patronus?

[prongs]

No: HPFGUIDX 77847

huge snippage
Pruneau:
But he chose the Patronus, so I assume that very few people  (only
Lupin, Hermione, and maybe Snape, Ron and Sirius) knew it was a
stag. So IMHO, the Quidditch Patronus was just a (large) silvery
wisp of vapour.

Pruneau
Now me, silver stag:
I think it was corporial during the quidditch match.  There wasn't any interference from real or fake dementors so it may have been easier, but more importantly is this from Chapter 22, when Dumbledore's talking to Harry just after Prof. Lupin leaves.
    Last night Sirius told me all about how they became Animagi," said Dumbledore, smiling. "An extraordinary achievement -- not least, keeping it quiet from me. And then I remembered the most unusual form your Patronus took, when it charged Mr. Malfoy down at your Quidditch match against Ravenclaw. You know, Harry, in a way, you did see your father last night.... You found him inside yourself."
One of my favorite quotes, but apart from that it shows, imo at least, that Harry's patronus was distinct during the match.


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