C++ - strange time mesurements with and without if statement

I stumbled upon a strange duration difference between two equivalent increment syntax.

if(seed != state) ++i;

this notation measures 2.25 ms per 1048576 iterations

i += (seed != state);

and this 2.80 ms per 1048576 iterations.

Shouldn't the second notation be a bit faster than the first? It is to mention, that seed == state happens very rarely.

Thanks for your answers.