Overall goal difference is not the first rule to separate teams on equal number of points in the group stage of UEFA Champions League
http://www.uefa.com/M.../1630244_DOWNLOAD.pdf
- See Article 7
If two or more teams are equal on points on completion of the group
matches, the following criteria are applied to determine the rankings:
a) higher number of points obtained in the group matches played among the
teams in question;
b) superior goal difference from the group matches played among the teams
in question;
c) higher number of goals scored away from home in the group matches
played among the teams in question;
d) superior goal difference from all group matches played;
e) higher number of goals scored;
f) higher number of coefficient points accumulated by the club in question,
as well as its association, over the previous five seasons
So it mostly hinges on the matches played between the teams equal on points.
Chelsea & Valencia are level on points
There has been one match between Chelsea & Valencia – a draw
But Chelsea were away from home so according to rule c) there are at present above Valencia.
So if Chelsea draw 0-0 in the home match they will remain above Valencia.
According to rule c.
Same reason why Benfica are above Man Utd
They scored more goals way from home in the matches between them
B v MU 1-1 MU – B 2-2