C# rounding differently depending on platform?

Question

I have this tiny piece of code

double s = -2.6114289999999998;
double s7 = Math.Round(s, 7);
double s5 = Math.Round(s, 5);
double s6 = Math.Round(s, 6);

With Platform = Any CPU, I get

s7: -2.611429   
s5: -2.61143    
s6: -2.611429   

With Platform = x64, I get

s7: -2.6114289999999998 
s5: -2.61143    
s6: -2.6114289999999998

Why? (Output copied from VS's Locals window)

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The whole piece of code is:

    private void btnAlign_Click(object sender, EventArgs e)
    {
        double s = -2.6114289999999998;
        double s7 = Math.Round(s, 7);
        double s5 = Math.Round(s, 5);
        double s6 = Math.Round(s, 6);
    }

Answer 1

The value -2.611429 cannot be represented using 64-bit floating point. When compiling in 32-bit mode the value will instead use 80-bit (extended precision).