battery voltages DO change all the time, jumping up and down, a little bit or a lot, influenced mostly by temperature, load and flaws. A carefully designed device firmware can make it seem the battery level is steady. Now you see for yourself, it is not actually steady. Likewise the reported level of what is maximum/100% or 75% is up to the firmware. New batteries don’t have to be 100%.
Also devices may have discrete reporting levels with nothing available in-between those levels. The available levels vary by device.
for IDE I don’t know.
looks normal to me for the past 5 years.