This is because your x value is different for the point on the bottom compared to the point on top. Your x axis range is so small, that you are able to see this difference (
=1.0101e-10).
=1.0101e-10).I guess technically, it has more to do with the ratio of 
to the range of x. You could try adding more data points to t to make it less apparent.
to the range of x. You could try adding more data points to t to make it less apparent.t = linspace(0,10e-9,1000);
x = square(t*2*pi/(10e-9));
Y_max=x*370.0;
Y_min=x*323.4;
hold on
plot(t,Y_max);
plot(t,Y_min);
