Fuel Guage Readings are undercounting voltage

We are using E Series with 3.7V 15Ah 1s3p Li-Ion batteries and solar charging through V-in. For some reason, some of our units will significantly and consistently under count the battery voltage. The fuel gauge will read a ~4.0V battery as ~3.7V. The issue seems to occur both when solar panel is getting input and at night What is the cause of this and how could we get more accurate readings?