There's selection bias at any time scale. Those who manage to sell their property at all are getting better prices than those who fail to do so.
The point is that you can't calculate a probability of making +15% within 90 days just by looking at how many of the houses sold within 90 days were sold for +15%. Or any +X%. Or any time period.
If you really want to calculate such a probability, you need to buy a bunch of houses at random, then try to sell them, and then see how much you made and how long it took. The data used in the post is not suitable.
