Q:

What is the reason behind the difference in Results?

let x = [1 2 3;4 5 6] then why norm(x,2) is different from norm(x(:),2)
norm(x,2) = 9.5080 and norm(x(:),2)=9.5394.
I have run this program in Matlab R2012a.  

A:

As defined in Matlab help for norm, the returned value is "The largest singular value" for matrices, and sum(abs(A).^2)^(1/2) for vectors.
Additional reading:
wikipedia - matrix norm

A:

x(:) is a vector, see what Matlab returns if you simply type that at the command-line. x is a matrix.  The 2-norm of a matrix and the 2-norm of a vector are calculated in different ways, in general the 2-norm of the vector of the elements of an array will not be the same as the 2-norm of the array.  For details see good old Golub and Van Loan.
Why are the 2-norms of matrices and arrays different ?  That's maths and therefore off-topic here on SO so I daren't answer.

