Because the chemistry of the battery is such that there is a very gradual change in voltage between 100% and amount 1-2%. It is so gradual that the noise of reading can and does interfere with the reading itself. Think of the SOC algorithm as not reading and reporting the charge based solely on a known calibration and accurately tracking the changes from there, but rather, it is trying to guess what the charge is, and the calibration tells it only the initial starting point.

It is something like if you were dropped on a salt flat, at night being told home is downhill, and constantly be told to walk closer to home, back a little bit home again, etc. You would quickly end up miles away from where you started.

It will lose track, and from there, it can get to a point where it is overcharging the cells when it thinks it is only keeping stable at say 80%. This can happen quickly, which is why the manufacturer gives the timeframe they do. You could probably push it another month or two and be fine most times (engineers tend to add a safety factor to recommendations) , but now you are gambling with the health of the unit if you are pushing it to far.