DH: The silver doe as Lily's patronus?

[clio44a]

No: HPFGUIDX 172256

Finally, finally some serious discussion about DH. *clio bounces up
and down in her chair*

Right. I'm still digesting DH, but this, trivial as it is, is right
now my most burning question: 
In DH 'The prince's tale' we learn Snape's patronus is the doe. In 'A
flaw in the plan' Harry tells Voldemort that Snape has the same
patronus as Lily Potter. 
Odd. How does Harry know? Am I overlooking something? I can't shake
the feeling that there was a conversation about this edited from the
book. (BTW, I am reading the British edtion, since the German
translation won't come out before October)
Someone please give me a hint.

Clio