The term waterproof doesn't always mean what people think it means. Our tendency is to think in absolute terms... that waterproof means it is waterproof no matter what. Turns out, that whether or not something is waterproof often does depend on something(s). That something in this case is pressure (could be temperature, soak-time related, etc as well).
One simple way to get a rough idea on the pressure issue is to take a clue from what tight-wad said about the water bucket. Put a piece of the material under a faucet and try to make a water balloon out of it. If water doesn't seep through, try rubbing the bottom or squeezing the balloon some. You will see a difference in tyvek, silnyon, rip-stop nylon with and without DWR, various rain gear, pack covers, plastic drop cloth, etc. in how easily, if at all, water seeps through.
If it seeps through a bit on that test, then it will be an issue as a tent floor if water is under the tent because your body weight, especially when you are on your knees and concentrate you weight in a smaller area, will generate a lot of pressure that is trying to force the water through the tent floor. Tent floors by the very nature of how they are used, need to be waterproof at a higher pressure than the top of the tent.
If this waterproof versus pressure seems odd, it isn't and most of us have dealt with it with watches for many years where they specify at what depth various watches are waterproof. Water has weight and the deeper you go, the higher the pressure because of the weight of the water on top of you.