Furthermore, for sterilization 70% is more effective because the other 30% is water, which helps ensure everything is exposed to isoprop for long enough and bacterial cells take in the isoprop and die (because water passes through the cell membrane, taking isoprop into the cell with it), rather than ‘hunkering down’ and surviving until the solvent is gone
However for cleaning electronics, the water content is bad because it does not dry quickly and can cause corrosion, so 99% is needed
So the percentages have varying uses and should be chosen based on the task at hand