In calculating the cell potential from the electrode potentials, some teachers and books reverse the sign of the electrode potential of the reductant of the reaction and then add the two potential values when they add the two half reaction chemical equations.
Although this gives the correct answer it is scientific nonsense.

Reversing the sign of an electrode potential has no physical meaning, and the cell potential IS the DIFFERENCE of the electrode potentials.

Also be very careful when reading a table of electrode potentials.
Many books, etc. still give tables with the most positive electrode potential at the top
as the table below from an examination paper shows.

Half-reaction E / V
O2 + 4H+ + 4e¯ ® 2H2O 1.23
Br2 + 2e¯® 2Br¯ 1.10
Ag+ + e¯ ® Ag 0.80

I2 + 2e¯ ® 2I¯

 0.54
Cu2+ + 2e¯ ® Cu 0.34

Sn2+ + 2e¯ ® Sn

 -0.14
Zn2+ + 2e¯ ® Zn -0.76