One very language-lawyer way to look at it is that, even if the answer it gives is more mathematically accurate, it's still less correct in that it's not the value you literally asked for.

Imagine the analogue for integers. You might use code like this to round down to a multiple of 2:

    int i = 7;
    int j = (i / 2) * 2;

If a compiler optimized that to j = i then that's more mathematically accurate but less correct.