Of course it does. 0.1f is not 0.1. 0.1f is 0.000110011001100110011001101b in binary (which is actually represented internally in "binary scientific notation" as 1.10011001100110011001101 x 2^-4). Similarly 1.3f is not 1.3, it is 1.01001100110011001100110b. Add them together, and you get 1.01100110011001100110010b, which evaluates to 1.3999998569488525f.