[texhax] plain tex vbox question

[jw]

Hello,
I have the following two .tex files:

% ---- begin file 1 ----
The quick brown fox...
\bye
% ---- end file 1 ----

% ---- begin file 2 ----
\vbox to \vsize{The quick brown fox...}
\bye
% ---- end file 2 ----

The text of file 1 is shifted down on the page a few points, while the
text of file 2 is right up against the top of the page.
In other words, there is some space above the "T" in file 1, but none in file 2.

I don't understand why.
I have looked around the TeXbook and online, but haven't seen anything
obvious...

Can anyone explain this to me? I feel I am missing something fundamental.

Thanks
-John