Page number is wrong at start of page

This is a long story, whose sources are deep inside the workings of TeX itself; it all derives from the TeX’s striving to generate the best possible output.

The page number is conventionally stored in \count0; LaTeX users see this as the counter page, and may typeset its value using \thepage.

The number (that is to say, \count0) is only updated when TeX actually outputs a page. TeX only even tries to do this when it detects a hint that it may be a good thing to do. From TeX’s point of view, the end of a paragraph is a good time to consider outputting a page; it will output a page if it has more than a page’s worth of material to output. (Ensuring it always has something in hand makes some optimisations possible.) As a result, \count0 (\thepage) is almost always wrong in the first paragraph of a page (the exception is where the page number has been « forcibly » changed, either by changing its value directly, or by breaking the page where TeX wouldn’t necessarily have chosen to break).

LaTeX provides a safe way of referring to the page number, by using label references. So, rather than writing:

Here is page \thepage{}.

you should write:

Here is page \pageref{here}\label{here}.

(note: no space between the \pageref and the \label, since that could potentially end up as a page break space itself, which rather defeats the purpose of the exercise!).

Source: Page number is wrong at start of page