Asm Substraction Question

I just want to know why when i add the to float together, that give me the right aswser and not when i substract them? Why? float SomeFunc(float x) { x = 0; static float Num = 2.4; _asm{ mov eax, dword ptr x //move x into eax //add eax, dword ptr Num //add eax by Num(Work) sub eax, dword ptr Num //sub eax by Num(Don't Work) mov dword ptr x, eax //move eax into x } return x; }