Ron's Rat.... or was it???

[Scott <insanus_scottus@yahoo.co.uk>]

argh! can't remember who, but *someone* wrote:
"The way I look at it, the whole thing is a wonderful bit of 
misdirection. Why should only the HP books be entitled to such 
narrative devices? The movies are just as entitled to allow us to 
jump to conclusions and believe one thing while another is acutally 
the case..."

me:
This makes sense to me, and they could easily clear it up with some 
PoA dialogue along the lines of-
HERMIONE: But Ron, your rat *can't* be an animagus! I mean, you've 
transfigured him before, haven't you, and you that's not possible 
with a human.
RON: No, I've never done magic on Scabbers. McGonanagall always 
gives us animals in Transfigurations...but it doesn't matter does 
it? I mean *look* at him!

...or something along those lines. However I also doubt that they'll 
take precious screetime for it, or that they'll even remember. 

Yet I'm still confused, which, though I'm oft times confused, does 
have a point...*why* have we decided that Rat!Peter couldn't 
transform? Is there canon evidence that I'm overlooking as to the 
impossibility of Human to nonliving transfig? 

Sorry! (being dense),

Scott