Car battery voltage readings different with no load - normal?
Yes, this is perfectly normal. A battery has an internal resistance. When you add a load on to it, it becomes a resistor divider. 

For example, assume you have a 12V battery and it read 12V exactly when measuring it. Imagine this battery has a 0.2 Ohm internal resistance. You then add a load of 10 Ohms. 

Use the voltage divider equation [Rload/(Rbat+Rload)]*Vbat = 11.76V

This is why you get a variation in voltage measurement when you measure it with and without a load. Other environmental factors can play a part, such as temperature, but the main culprit in many cases I think you will find is the internal resistance.