Floating Point Monotonic Property

I was reading a book (CSAPP) . In the book it is written that-

floating point addition satisfies the following monotonicity property :
if a>=b then (x + a) >= (x+b) for any value a,b and x other than NaN. This property of real (and integer) addition is not obeyed by unsigned or two's complement addition.

How floating point obeys it?
Why unsigned or two's complement addition doesn't obeys it?