There are many things, such as the surface charge effect peukert and internal resistance meaning voltages change and drop or rise over time or fall chemicaly under load.
But the also RISE frequently as you take power (Ah OUT) of the battery...
So here I am taking 4A continually over three quarters of an hour. While the voltage initially fell fast, and the battery guage told me I had lost 3 lights... Then it rose over time as we DISCHARGED the battery. So now I took 3Ah out and the battery gauge is tlling me its now fully charged again...
Volts do NOT = state of charge even on a simple lead battery! Or they would be falling, not climbing here:
Whats really happening is that the battery has an internal resistance, and so as you load it up slightly it warms up. And all chemical reaction work faster as temp increases. This may only be fractions of a degree. But it means higher terminal voltages and a lower internal resistance. So the voltage increases as you use up the battery initially. But the battery meter doesnt know this. And it can never know the core temperature or figure out the difference betwen voltage drop due to discharge, or load or temp changes, or pukert, or previous 24 hour surface charge history... So they are useless. They are just feelgood devices to humor the driver.