Social Blade's future predictions are not all that hard to understand. It is all linear. "It took you 6 months to get 50 subs? So we think it will take 6 months more to get 100." It doesn't take into account that you could go exponential and actually end up with way more. When I first started my channel back up, it said that I might hit 40 subs in 5 years. Well after getting 5 new subs since the beginning of the month, it think I will hit 150 in that time. It just averaged out the past 6 months and looked at that growth rate. If it see a growth rate of 5 per month for me then it will say I should have 30-35 subs by the end of the year. So it really just takes the latest trend of your channel and maps you out linearly.
So for you, it says that you are gaining 13 subs a month. If you continue to gain 13 a month, then you will hit 100 sometime in late February or early March but lets say you were to start generating traction next month and got 20 a month, well you could hit that milestone in early January. It obviously can't see that you are going to upload more videos and gain more so it cannot trend that but if you continue to upload more quality videos, then you will gain more and it will start to reflect that.
Personal Tip: Only look at Social Blade like every 2 weeks or so. That way you can get a more realistic approach. I have an excel spreadsheet with my subs that I update every 2 weeks so I can personally model myself out. I can see the trends for my channel and determine that I will hit 100 around April if I continue to put out videos. The only thing that I don't know is how a new series will do for me so I can't model that out until 2 weeks after it hits.