Python Recursive Function Strange Behavior

I have written Longest Common SubSequence in Python using recursion with memoization:

def print2d(table):
    print '\n'.join([''.join(['{:4}'.format(item) for item in row]) for row in table])

a="123"
b="213"    
m = [[-1]*len(b)]*len(a)
def lcs(i,j):
    print i, j
    print2d(m)

    if i== -1 or j == -1:
        return 0;
    if m[i][j] != -1:
        return m[i][j]

    res = 0
    res = max(res, lcs(i, j-1))
    res = max(res, lcs(i-1, j))
    if a[i] == b[j]:
        res = max(res, 1 + lcs(i-1,j-1))
    m[i][j]=res
    return res

print lcs(len(a)-1,len(b)-1)
print2d(m)

All those print statements are there because I didn't get the correct result and decided to take a look at how the algorithm works. What I found surprised me. If you run it yourself you can see the tables printed and they look ok until:

0 -1
  -1  -1  -1
  -1  -1  -1
  -1  -1  -1
-1 0
  -1  -1  -1
  -1  -1  -1
  -1  -1  -1
0 -1
   0  -1  -1
   0  -1  -1
   0  -1  -1
1 1
   1  -1  -1
   1  -1  -1
   1  -1  -1
1 0
   1  -1  -1
   1  -1  -1
   1  -1  -1
0 1
   1  -1  -1
   1  -1  -1
   1  -1  -1

Why all of a sudden on the step 0 -1 the whole first column became 0 ? So, I quickly created C++ program doing exactly the same thing the same way:

#include <iostream>
#include <iomanip>
#include <string>
#include <cstring>
using namespace std;
string a = "123",
       b = "213";
int mem[1000][1000];

int lcs(int i, int j) {
    cout << i << " " << j << "\n";
    for(auto i = 0; i < a.length(); i++){
        for(auto j = 0; j < b.length(); j++){
            cout << setw(4) << right << mem[i][j];
        }
        cout << "\n";
    }
    if (i == -1 || j == -1) {
        return 0;
    }
    if (mem[i][j] != -1) {
        return mem[i][j];
    }
    int res = 0;
    res = max(res, lcs(i, j - 1));
    res = max(res, lcs(i - 1, j));
    if (a[i] == b[j]) {
        res = max(res, 1 + lcs(i - 1, j - 1));
    }
    mem[i][j] = res;
    return res;
}
int main(){
    memset(mem, -1, sizeof mem );
    int r = lcs(a.length()-1, b.length()-1);
    cout << r << "\n";
    return 0;
}

And it works as expected. Corresponding tables look like this:

0 -1
  -1  -1  -1
  -1  -1  -1
  -1  -1  -1
-1 0
  -1  -1  -1
  -1  -1  -1
  -1  -1  -1
0 -1
   0  -1  -1
  -1  -1  -1
  -1  -1  -1
1 1
   0  -1  -1
   1  -1  -1
   1  -1  -1
1 0
   0  -1  -1
   1  -1  -1
   1  -1  -1
0 1
   0  -1  -1
   1  -1  -1
   1  -1  -1

I am puzzled why Python and C++ code that are not that different produce such drastically different results.

Am I missing something about how recursive functions in Python work? Or is it because in Python I use list of lists and not 2D array as in C++?