On Saturday, I checked the my simyo data to ensure that I was not using too much data and saw that I had used 4 gigabytes so far for the month, so I used my hotspot for a short time to let my daughter watch a show.
However, it appears there was a large delay in the Internet data remaining, and I had actually much less data remaining.
This seems unfair to me, as I specifically checked the app to see how much data was remaining and made a choice based on the information provided.
As an another example, right now, on my wife's phone, it shows her data usage for yesterday as 0 megabytes on the Mijn Simyo app, but I know that her data usage was non-zero.