replacement - Why does using ReplaceAll twice produce a different result?

Why does this:

Solve[a x^4 + b x^3 + c x^2 + d x + e == 0,  x] /.
    {2 c^3 - 9 b c d + 27 a d^2 + 27 b^2 e - 72 a c e -> F}

produce a different result from this?

Solve[a x^4 + b x^3 + c x^2 + d x + e == 0,  x] /.
    {2 c^3 - 9 b c d + 27 a d^2 + 27 b^2 e - 72 a c e -> F} /.
    {2 c^3 - 9 b c d + 27 a d^2 + 27 b^2 e - 72 a c e -> F}