Optimization test: -= faster than += ...?

1

I did a performance recently…


var a = 100;
for (var i:int=0;i<10000000;i++){
    a += 1;
}
trace(getTimer());
//traces 410

and then…


var a = 100;
for (var i:int=0;i<10000000;i++){
    a -= -1;
}
trace(getTimer());
//traces 280

Both are doing exactly the same thing, but in different ways. The -= method did it much faster.
Can it be true?