What are you trying to find with your max? Do you want to find the max, ignoring the structure of your code containing 2 columns or do you want the max per column?
In the first case use
max( a(:) )
In the second case the 1x2 result array that you get will contain the max from each column which will likely be on different rows to each other so this cannot then be compared against the original 'a' matrix.
