about 3.2 Memory addressing modes

[einQimiaozi]

Why use wrapping_ add() method?
For example:
pos is 200,self.register_x is 111
The result of adding them is 311,but using wrapping_add(),the result of the add method is 000000010011011, i.e. 55
The addressing range of NES is 0-65535, so why? Isn't that an error?
thx