Due to the variable nature of events, calculation of gems gotten through events is at best an approximation. To make the calculations simpler, it assumes that each month has 2 events, ending on the 1st and the 15th, and events alternate between token events and score matches (and medley festivals if on JP.) Also, if you are tiering, it assumes that you will spend some gems to tier, and will use an estimate based on your average tier, but this will only be an estimate.)