Reading GPU Usage Graph

I am using EVGA's Precisionx version 16 to monitor the usage of my graphics card. On the graph there is a max value and a min value. For one thing I do not understand why it's not just 0 to 100 for percentage usage. First the scale was 20-35 and then later while gaming it somehow changed to 0-44. How can I get a clear picture of the pure percentage of GPU usage?