Now 69.3GB are used. Average this month is 16.7GB. Looks reasonable.
Please clarify your question.
It even states that for yesterday the disk usage was about 40Gb. This makes sense too, because the disk filled over the day and so the average usage will slowly increase.
The “low” average month value is, because you just started your node yesterday
I assume that you are refering that for the 28th of Feb you are seeing 0B usage (with the 40GB spike from the 27th).
The graph you see on the left, average disk spaced used, is not calculated locally, rather they come from reports from the satellites.
This in turn means that until your node receives these daily reports, it will display 0 usage for the day. Usually they are received by the end of the day, but these tasks failing (and therefore not receiving any data for the day) is rather common, which leads to apparent low/none space usage. While this does mess up the graph and average usage for the month in the nodes, it does not have any impact on the payouts, as they are calculated in a different manner.
Therefore, to summarize, the reason you see that big drop with 0 usage is because you have not received the usage report yet.
Keep in mind that due to how garbage collection works, you will most likely encounter a difference between the actual usage (right graph), and reported usage (left graph, when reports are received correctly). This difference should generally be less than 10%.