you are trying to use the past to interpret the future and you expect the markets to move in the same exact way and things do not work like that
If you want to have a statistical calculation you need to have a model to have a reference on how the market will move and we have a small data available as this is a new market, we cannot expect the market to move the same all the time but it is the only reference we can have on how the market used to perform in the past during those situations 
.Past behavior does not predict future behavior. Btc could go well hit 20 k tomorrow or dump to 1k.
It is highly unlikely to happen just like that. The past behavior will give you an idea on how the market will perform, it cannot be an accurate prediction but you can use that as a reference.Yes, there is no guarantee that things that happened in the past months or years can also be happening next year in almost identical manner but data we can gather from the past can be serving as a guide on probabilities and that is why we can be creating some models. It is like predicting the future based on the past though again there is no formula that can exactly tell us what can be though this is much better than to just wildly guess the possible outcome. We have to always remember though that Bitcoin has this habit of being unpredictable at times so we should not be shocked to see some good and even bad surprises along the way leading to the scheduled having in 2020.