It seems you are getting tripped up by the fact that the code below results in two identical lines
y1=step(H,x);
y2=step(c2d(H,0.1),x);
plot(x,y1);
hold;
plot(x,y2,'r')
but this code does not
close;
step(H,x);
hold;
step(c2d(H,0.1),x,'r');
There is no error here. As you noticed y1 and y2 are identical, as they should be, because your discretization step of 0.1 is the same step you use for defining x.
The reason why the first batch of code does not produce the same graph as second batch of code is that function plot always interpolates between data points. It is not the right function to use for plotting the step response of the discrete-time transfer function. For that, use the function stairs:
close;
plot(x,y1);
hold;
stairs(x,y2,'r')
Hope this helps.
