Thanks guys, this clears it up a bit. So i guess the general idea here is when the runway is contaminated, however not slippery, it will only provide extra drag, which slows your acceleration, but helps you brake. That's why you choose a higher V1.
However, now my next question, when is a runway contaminated and not slippery? Do slush and standing water only provide extra drag? I'd think it'd make the runway slippery to...