013 starts with a 0, so is an octal, its decimal value is
1*8 + 3 = 11
And of course, 11 + 1 is 12, since the value of var2 is printed in decimals, not in octals.
When a number starts with a zero, it's assumed to be written in base 8 (octals). When it starts with 0x, it's assumed to be written in hexidecimals (base 16).