How does Debian keep Packages.gz consistent over different sites?

[Try KDE <trykde@rogers.com>]

Lets say I have two lines in sources.list that are almost identical 
expect for the http sites are different. And now I issue a "apt-get 
update", and site1 tells apt "the latest version of testing/vim is 1.3", 
while site2, which may not be up to date, tells apt "the latest version 
of testing/vim is 1.2". So now what apt is supposed to do? Or somehow 
this never happens? If it never does, how debian "pushes" the changes to 
mirror sites to make sure they are up to date at any moment?

Thanks,
-tk.