gradient of gradient and laplacian of one-dimensional vector

I have a 1-dimensional vector which is:

p = [1 3 6 10 16 18 29];

laplace = del2(p);
px= gradient(p);
pxx=gradient(px);

Since it is one-dimensional, laplacian of the vector must be equal to gradient of gradient. However they give completely different results.

laplace =    0.2500    0.2500    0.2500    0.5000   -1.0000    2.2500    5.5000

pxx =    0.5000    0.7500    1.2500    0.2500    0.7500    3.5000    4.5000