The history graphs in iStat Menus dynamically change their scale, depending on the content they are showing. This is done to ensure peak values always fit.
For the hourly history graphs, each sample shows the average for a 6 second period. Each sample in the day history graphs shows the average for a 144 second period. Because values are averaged for each sample, it is common for peaks to drop, unless the peak is sustained for a very long period.