The calculation method has been updated in v121 firmware to minimize the error between the calculation and the frames actually taken. As you well know, the compression ratio of JPEG image is largely affected by subject and scene. The previous firmware v021 tends to count the remaining number of frames a little larger than those will actually be saved in the card, thus in the worst case, the remaining number display would have dropped suddenly when some high-frequency images were saved. In v121, the calculation method has been changed to add more accuracy.