Is it so short that there is an intermediate tab stop to the right of it? If
so, then the tab Word inserts for the page number is going to that tab stop
instead of the one at the right margin. There's not much way you can fix it
except to either move/delete the tab stop or make the entry longer (if it's
a left-aligned heading in the text, you could add a series of spaces, ending
in a nonbreak space), or you could add a tab character after the heading and
add the \w switch to the TOC field to preserve the tab.